How common are reflexive pronouns in natlangs?

Generally speaking, you need a reflexive pronoun of some type if not having one causes ambiguity in who did what to whom. Assume English had no reflexives. "I tossed the ball to I" and "You tossed the ball to you" (assuming singular "you") are quite understandable as one person tossing the ball to themselves, thus no reflexive needed. "She tossed the ball to he", acceptable. But suppose you had "She tossed the ball to she", and it's been previously established there is more than one woman present. Did person A throw the ball to Person B, or did she toss the ball up in the air and caught it herself? There's no way to tell. An example like that is why German only has a distinctive reflexive in third person; first and second person don't have that same ambiguity.
You can get rid of the reflexive entirely if your verb system, to use one possibility, allows a given verb to indicate if it takes a direct or indirect object, or if it does not. Let's say that English verbs used the affix a- to indicate the verb is ditransitive (it has both a direct and indirect object) and no affix to indicate purely transitive (it only has a direct object).

"Mary and Susan were on the field. Mary had the ball. She a-threw the ball to she." - Mary threw the ball to Susan.
"Mary and Susan were on the field. Mary had the ball. She threw the ball to she." - Mary threw the ball up in the air and caught it herself.

Under a system like that, English wouldn't need a reflexive pronoun because the verb would make it clear whether the subject was performing the action on themselves, or using themselves as the indirect object, or whether something else was the recipient of the action. One could argue the affix a- is similar to the single Icelandic reflexive pronoun that's used for everything.
In Inuktitut, verbs that are are equivalent to English intransitive verbs use one suffix attached to them to indicate the subject:

pisuk to walk
pisuktunga I am walking
pisuktutit You (singular) are walking
pisuktuq He/She/It is walking

Specific verbs, equivalent to transitive verbs, use a different set of suffixes that indicate both subject and object:

qukiq to shoot something
qukiqtagit I shoot you
qukiqtara I shoot him/her/it
qukiqtarma You shoot me
qukiqtait You shoot him/her/it
qukiqtaanga He/she/it shoot me
qukiqtaatit He/she/it shoot you
qukiqtanga He/she/it shoot that other he/she/it

Here's where the reflexives come in: if you don't use the second set of suffixes, but instead use the first set, then the verb becomes a reflexive: the subject did something to themselves.

qukiqtunga I shoot myself
qukiqtutit You shoot yourself
qukiqtuq He/She/It shoot themself

Attaching the suffix that goes on intransitive verbs onto a transitive verb turns that verb into a reflexive.
So, if your verb conjugation allows that sort of thing, then you can get away with not using reflexive pronouns. The verb indicates it.
On the other hand, if your verb doesn't allow you to do that, then you need reflexives in order to remove the ambiguity when you get into situations where it's not clear who is doing what to whom.
Another way: go back to that first English example I used. Suppose English had multiple third person singular pronouns, such that you didn't just have "she", but "she-1", "she-2", and perhaps more. Then what does the example look like?

"Mary and Susan were on the field. Mary had the ball. She-1 threw the ball to she-2." - Mary threw the ball to Susan.
"Mary and Susan were on the field. Mary had the ball. She-1 threw the ball to she-1." - Mary threw the ball up in the air and caught it herself.

No ambiguity. The context makes it clear that Mary is she-1 (Mary had the ball, she-1 threw the ball, therefore Mary is she-1, so Susan is she-2).
A third option is context. You might have a language where you do need to gather more information in order to determine what's going on.

"Mary and Susan were on the field. Mary had the ball. She threw the ball to she. Susan threw it back." - Mary obviously threw the ball to Susan.
"Mary and Susan were on the field. Mary had the ball. She threw the ball to she. Susan then asked for the ball." - Mary apparently didn't throw the ball to Susan, since Susan asked for it after it had been thrown and caught.

So, three possible ways to get away without reflexive pronouns. So it's not a matter of how common or uncommon it is, but does the grammar of the language allow you to get away without them, or is it necessary because there's no other way of providing the same information?


It's worth noting that there are PLENTY of word frequency lists around, but almost no one ever bothers to even try compiling cross-linguistic ones (for reasons that should be pretty obvious). Lists for English, furthermore, hardly ever even attempt to separate different word categories (again for reasons that ought to be obvious).

The Conlanger Lexipedia is based off a 1500 roots list (with some caveats) compiled from a 1.1 million word corpus of science-fantasy writing. You can count the verbs yourself... once you decide what counts as a verb (the list can't tell apart verb vs. adjective vs. noun uses, and the commoner an English content word, the likelier it is to drift between noun/verb/adjective).

The top 200 words (page 36-37 of the book) have the following roots that are "primarily" verbs:

  go, will, know, see, come, like, would, could [end of top 50 words, could is 49]
  look, give, seem, must, call, think, make, ask, work, run, mean, may, wear, feel, want, stand, hear, open, should, end, keep, sit, need, turn, move, try, begin, place, close, talk, can, get, take, tell, find, speak, might, let, hold, fall, bring

Be mindful of the biases introduced by an English list: the English auxiliary system, in particular, wouldn't be reproduced in any other languages as the past forms (and very often the main forms!) would be regular verbal inflections instead of separate forms. Arguably, I shouldn't have included the auxiliary verbs at all, really:strictly speaking these verb have no infinitive form to begin with.


By what criteria can we say that a conlang has a functional speech community?

Here are some ideas, which when taken together would give a good idea whether a language has a strong functional speech community.

If a conlang has thousands of fluent first language (L1) speakers, that would be clear evidence, but even Esperanto, the most spoken conlang, has only up to 1000 Esperanto speaking families. Ideally those L1 speakers are able to meet and speak with each other regularly.
Even if the speakers are second language speakers, if there are so many speakers that distinct dialects have formed (without that being part of the design from the beginning) then that evidence of language change is evidence itself of healthy speech communities.
If there is friction between speakers of the language and the language's academy or regulators then that shows the language is starting to take on a life of its own. If the language regulators have ever had to make concessions to the speakers, then that shows the speech community is healthy.


What decides where is the break between syllables?

I think as a general rule, a syllable comprises some combination of the following kinds of one or more of the sound types of your language:  vowel, liquid, consonant.

You've decided on (C)CV.  And if there are two C then one must be a sibilant and the sibilant must follow a stop. So you can have syllables like sa, ta, tsa  but apparently not sta.  You might then have a words like tasa & satsa & tsatsa.

In English, we'd probably break that into SAT-SA.  Your language will likely differ, because everyone knows that a syllable can't end with a consonant!  So, they'd much more likely break the word thus: SA-TSA!

I would argue that what little you've described of your language and its rules speaks in favour of E-PSE rather than EP-SE.  The only thing you "did wrong" was to apply the English rule rather than the rules that apply to your invented language! The second thing you did wrong was to second guess yourself based on the English rule!


Grammar concepts required for every conlang

Is there a method to make sure that a constructed language contains all the necessary grammar concepts, e.g. to make sure that a concept like genitive case or a construct state is present?

The primary issue here is trying to find out what is necessary for a language to function; not even 'Chomskyan tenets' like recursion (or broader concepts like classifiers and numbers) seem to be obligatory, and language in essence seems to boil down to a glorified predicate machine with parts tacked on by Broca's area — and so, the answer is, for now, no: we'd very much like to see a way of doing this, and it would be truly groundbreaking in linguistics, seeing as it's a major focus point for many researchers in many fields of philosophy, linguistics, psychology and neurology.


Where should I start planning my grammar?

It would probably be a good way to start with the features you want to express through grammar. Many features can be expressed in different ways, lexically, through morphology, or through syntax. For example, 'subject' could be done through a particle, an affix, or through the position in the sentence.
You first come up with the list of features, and then you look at the inventory of forms in the grammar. Do you want a complex morphology? English, for example, marks very few features through morphology (plural for nouns, 3rd person singular for verbs, tenses, …) and a lot through word order or by adding auxiliaries.
The inventory will define the shape of your language; you can also start with that and then assign features to it. But be clear what kind of features you want to be able to express – not every possible distinction needs to be expressed in language. Do you want to mark the gender of the subject in the verb? Perfectly possible, but probably not very useful, unless your language is for a very gender-segregated society.
Obviously, the more features you want to express, the more complex your grammar will be. As an extreme example, toki pona pretty much only uses word order and a few particles to distinguish between subject/verb/object, and ignores number or gender markers. In Esperanto most work is done by the morphology, which encodes the role in the sentence, gender of nouns, number, and even a range of semantic relationships (eg "room where you do a particular action").


Is there existing terminology for distinguishing multiple imperative moods?

I don't think there is any really reliable cross-linguistic labelling system that would include all of these. There are terms for most of them, but they're often used for only a few languages, and perhaps not very consistently. For someone who is created a conlang, they have flexibility then to adapt these terms for their language, however the important thing is to clearly describe in detail the full semantics of the language, rather than just assigning labels.

The broad category for all of these is deontic modality, which covers the modality of how the speaker thinks things ought to be. Within deontic modality there are many subtypes. The SIL Glossary has a very useful section giving a hierarchical description of deontic modality.

English actually has a very large number of modal verbs, with subtly different senses, so with examples from English we can illustrate a lot of the subtypes, even though they're usually not given clear labels. The English modals are however very complex, most of them being used in different contexts to mean very different things.

The imperative is actually one of the least marked ways of issuing a directive/command. In English it is a structurally different way of forming a sentence, but in other languages it may have a more regular structure with a particular verb form that fits into a wider affix paradigm.
Should is usually weaker than the imperative, giving a recommendation. Possible labels include hortative and propositive.
Have to, need to and must can all be stronger than the imperative, but they also often imply that the person giving the instruction needs to plead to some extent with those they are instructing; an autocratic dictator would never tell their servants that they need to do something, they would simply tell them to do it (with an imperative sentence.) Labels include precative for requests, and directive and obligative for commands that emphasise obligation.
When will or shall are emphasised and stressed in English they are used to indicate a serious instruction, which sometimes sound almost sinister with an implied threat for those who disregard them: "You will do this... (or else!)" The label commissive is used for threats, but not exclusively, as it's also used for promises.


In fact, each language has its own punctuation rules that have to be learned. German is very different in its punctuation rules from English (relative sentences always require a comma, dependent clauses are separated from the main clause by a comma, but no "Oxford comma" and no comma after sentence-initial adverbials. Studying the rules of punctuation for some non-English languages should give you an idea of how things can be regulated differently.
I also learned that Russian and Kyrgyz and probably other languages have more ellispis-type multiple punctuations like !.. and ?.. and even !?. and ?!..
And than there are creative extensions to punctuation like the interrobang ‽, its inverted counterpart gnaborretni ⸘ and lots of proposals for an Irony mark.


Syllabic restriction/word boundaries set by IPA? Or it is decided by language creator

The phonotactics of a language, what you have described, is the silent partner of phonology. The analogy I like to use is that a language's phonology is its periodic table (with individual phonemes as individual elements), while its phonotactics are the entire rest of its chemistry. Higher level stuff, like morphology, semantics, pragmatics, are things like biology, psychology, sociology.
Just like every language has its own phonology, every language has its own phonotactics.  There are languages that can put /ŋ/ in onset (for example, Vietnamese) or /h/ in coda (Arabic). There are languages that can't make /s/ clusters, or any clusters for that matter (Hawaiian).
The IPA is the "Grand Unified Periodic Table of Linguistics"; it's stated purpose is to label every sound used in a human language, and does not restrict their arrangement in anyway. There is no "Grand Unified Chemistry of Linguistics"; phonotactic laws are usually described in plain language, although there is a notation to succinctly describe allophonic variation (that is, how phonemes are realized as phones in different environments) and phonemic evolution (how a language's phonemes evolve over time).


Is Tolkien's Dwarvish really based on Hebrew?

Yes.
According to this interview with Tolkien, he really did design it to be Semitic.  He says,

The dwarves of course are quite obviously,  wouldn't you say that in many ways they remind you of the Jews? Their words are Semitic obviously, constructed to be Semitic.

In other words, he did design their tongue to be very like the Semitic language, possibly since their history resembled that of the Jews.


Are there any resources to learn Solresol available in English?

You can find many articles at SiDoSi (sidosi.org)'s resources page or at this tutorial on blogspot. Also, I'd recommend a translation of "The second major book about Solresol, written by the other major contributor to Solresol, Boleslas Gajewski," also at sidosi.org over here.

Note that SiDoSi (sidosi.org)'s resources page has an error that requires you to click the [+] and then the link that appears because the direct links don't work and redirect you. Else see the beta version, without broken links.


From known languages, what are the expected features of a Generation Ship Language?

The idea that language would not change over 1000 years of travel in space is absolutely ludicrous. 1000 years ago was before Middle English existed. Massive amounts of language change can occur over such a long period of time. Even very conservative languages change significantly over such a long period of time. No language will remain unchanged after 1000 years. Based on natlangs, it's more likely than not that the language will not be mutually intelligible with its predecessor after 1000 years.

How exactly the language changes depends a lot on the details of the situation -- how multicultural is the ship? What is the lingua franca of the ship? How are other languages viewed relative to the lingua franca? If there is enough of a divide in where different languages are used (such as if everything for official purposes is in English but a great number of native, say, Spanish speakers speak Spanish with their families at home), you may end up with diglossia arising. If there are significant numbers of multilingual people on these ships, you can bet there will be a lot of borrowing from minority languages into the dominant language, at least in less formal contexts. As in real life, there may be stigma against using such borrowings.

Honestly, language change in this situation won't likely differ that much from language change anywhere else. A conlanger has a lot of freedom in how they want to deal with the diachronics in this situation. The only real difference is that you don't have any chance of outsiders suddenly arriving and bringing with them waves of loanwords. ...Well, at least, you probably don't.


Grammatical cases occurring only in conlangs

I think the best conlang to look for such cases is Ithkuil. http://ithkuil.net/04_case.html contains a list of all cases. My [short] research suggests that some of them (e.g. navigative for "noun relative to whose vector, arc, or trajectory of motion an event takes place" or allapsive for "amount of time that expected to pass between the contextual present and the time of a future event") aren't attested in any other well-known language. I may be wrong, but it seems plausible regarding Ithkuil's goal of expressing extensive detail.


What is a constructed variety of a natural language called?

That would be an a posteriori conlang, in contrast with an a priori one. Wikipedia. The process of changing a language through time is called diachronic conlanging.

An example of a very well-done a posteriori conlang would be Carisitt which has been developed as if it was a natural language deriving from PIE (that is, it is also an example of diachronic conlanging). Examples of an a priori conlang are abundant, for instance all of Tolkien’s works.


Valyrian - what living languages does it resemble the most?

It is probably impossible to say which language resembles Valyrian most, lacking a metric for similarity. But there are clearly identifyable influences of other languages, both constructed and natural.

Let's start with the catch phrase valar morghulis from GRRM: It just sounds like Tolkien's Elvish languages, with valar being a word from Quenya and morghul a word from Sindarin. Both Quenya and Sindarin have well-known influences from natural languages: Finnish for Quenya and Welsh for Sindarin.

Looking at High Valyrian, some Finnish feeling is still retained in the language: There is a distinction between long and short consonants as well as vowels, the case endings still have some Finnish feel (and a Latin feel, too, especially the vowel alternations), the phonotactics resemble Finnish. It is further away from Finnish than Quenya in its phonology contrasting voiced and voiceless stops and featuring the sound /q/. There is little Welsh in High Valyrian left, maybe the frequency of "the defining fantasy vowel" ae (Laura Wattenberg) can be traced to Welsh.

The 8-case system may be insprired by Languages like Russian or Sanskrit, with the commitative case being typologically rare, but occurring in Finnish again.

The four noun classes are a creative invention (but not far of, as natural language, especially Native American languages, are concerned) and I see no closer resemblence to Bantu languages (they have a lot of noun classes, but the noun classes are marked by overt prefixes that are also used as agreement markers).

The syntax with head-final relative clauses may be inspired by languages like Japanese.

P.S. While Peterson has stated some linguistic influences for the Dothraki language, I have not found a similar quote for Valyrian. So all the observations above are mine.


A modern approach to this problem is to generate the words automatically according to some formula describing allowed words. You can filter out words that are too close to already generated ones, but usually the randomness used by a computer program will be sufficient.

There are well-known examples of Conlangs with computer generated vocabulary. Loglan and Lojban used some vocabulary lists from natural languages as input for their vocabulary generating algorithm.


As a native speaker of American English from the middle of the North American continent, I have always pronounced it as /ˈænədu/, like Xanadu (the movie), but with out the leading /z/. Visit https://tophonetics.com for a close approximation (capitalization ignored, British and American options generate the same audio).


What are some real world data on the numbers of speakers of constructed languages?

There is some basic data at Ethnologue for Esperanto.
The Font of All Knowledge lists some statistics for other invented languages (Hungarian & Russian census data).

Not much, but a (small) start.


Naturalness of different universal quantifiers based on the size of the domain you're quantifying over

This is not inherently unnatural. As you point out, English "both" (and reasonable translations such as Spanish "ambos") easily fit the role of every(2) (I would argue that whether the noun is marked as singular or plural isn't particularly interesting here).
every(1) can be reasonably analyzed as a definiteness marking and is encompassed in most Indo-European languages by the definite article ("the", "el", "i", το), as shown by the sentence "The sun is shining." (This sentence can be used without explicitly introducing the sun into the conversation specifically because there is only one reasonable referent.)
In particular, I could see one set of forms evolving from the definite articles in a language with heavy number marking (i.e. "the (singular)" becomes each(1), "the (dual)" becomes each(2), etc.).
What I would be very surprised to see in a natural language is a fully consistent system of these, where each(), every(), and all() all cover the same numeric distinctions in the vein of the Esperanto correlatives.
I would also be surprised to see more than a couple of these in any language that does not make at least as many number  distinctions elsewhere.


Is there evidence that any natural languages were actually constructed?

Possibility? Of course, there is always a possibility, but rather remote one, if you mean a full featured well-known modern language.

The best candidates are "taboo languages", where the lexical items are replaced because of taboo/religious pressure. Perhaps the best known is Dyirbal, where a special version of the language is to be used in front of "tabooed" persons.

Though it probably does not go millennia into the past, and it's questionable if it should be considered a language, since it is just a vocabulary encoding.


21st century International Auxiliary Languages

The only legitimate examples from the Wikipedia article International auxiliary language are Nerrière's Globish (2004) and  Interslavic (2006). Both of these fail OP's criteria, since they are specific to one language branch. Other instances either predate 2000 (Kotava, Lingua Franca Nova), or were not intended as auxlangs (Lojban) or both (Toki Pona). To these, Wikipedia's list of constructed auxiliary languages adds Sambahasa (2007) and Lingwa de planeta (2010), both of which pass OP's criteria.
Ironically, the Interslavic wikipedia page says that "the language" dates from 1665—but what that actually means is the first Slavic-based auxlang, rather than Interslavic (initially Slovianski-N), which was created by Jan van Steenbergen in 2006.
(The Wikipedia page is also confusing in that it discusses the multiple variants of Slovianski and preceding and subsequent efforts together, rather than highlighting the lineage of the language now labelled Interslavic. But since work is ongoing to unify all those variants, that confusion is not surprising.)


What similarities does The Black Speech in Tolkien have to Hurrian?

Well, The Font of All Knowledge (a.k.a. Wikipedia) tells us several things:

There is not much similarity as far as phonology, as Hurrian seems to lack consonant voicing distinction (except in certain circumstances). BS clearly has this distinction.
There's not enough BS to determine much by way of grammar, but Hurrian seems to be extremely agglutinative; BS doesn't appear to be at all.
Hurrian seems to have more vowels.
BS has more consonant clusters.

Upon reading this analysis of Black Speech, I think it is clear that Professor Tolkien must have taken some words & grammatical forms into BS from Hurrian. For example:

-at < Hurrian -ed- formant of jussive/intended future in verbal form formant of future in verbs
-ûk < “All”, “completeness”; Hurrian -ok, formant with a meaning “fully, truthfully, really” in a verbal form.

Hurrian texts were known since discoveries in the 1910s & 1930s. A grammar wasn't published until 1941. Based on the Analysis, I think it very likely that Prof. Tolkien gained inspiration from the Hurrian direction. I'm not sure I'd agree with "strong similarity" based on the extreme paucity of BS evidence. More evidence, a lost grammar, those would be more convincing one way or the other.


Did a majority of followers of Ido indeed switch over to Occidental?

I'm as pro-Occidental as they come (it's the only auxiliary language that I support) but I came across that article in 1928 during my typing up the archives of Cosmoglotta and that article is nothing more than Ric Berger wishful thinking. He unfortunately spent just as much time attacking Ido as he did supporting Occidental, and kept on proclaiming it to be moribund and ready to collapse for some 20 years.

Here's the article in question that he wrote in 1928 on the Ido conference from which he concluded that Ido was on its last legs:

http://cosmoglotta.pbworks.com/w/page/132587775/Cosmoglotta%20A%20053%20%28oct%201928%29 (Title: Raport del Occidental-Observator Ric. Berger al Association «Cosmoglotta», Wien.)

So the answer is no. However, it's also true that the majority of the Occidentalists at that time were once Idists, from Berger to Gär to Ramstedt to Pigal to de Guesnet and all the rest. There was definitely a large influx from Ido to Occidental, and Occidental as far as I can tell was the most active auxiliary language after Esperanto by the late 30s and into the 1940s. Matejka and Berger had a lot of back-and-forth in the 1920s to early 1930s until Matejka decided to move over to Occidental, which he stayed with until his death in the late 1990s. He's probably the most prominent example of a major Idist who went full in on Occidental. But Berger hadn't taken into account the other source for new Idists, namely new users who weren't satisfied with Esperanto but still liked the overall concept (i.e. the people that thought that Zamenhof almost got it right).

Anyhow, I removed that sentence from Wikipedia last year when going through the Cosmoglotta archives because it certainly isn't backed up by anything besides Berger's wishes.


Is there a language that meets these criteria?

I think it should be toki pona. Reasons:

semantically close words must share a common affix (not like biology and mathematics in English which are sciences but do not have the same suffix)

In toki pona, every science has a compound noun word beginning from sona. Biology is sona jan and Mathematics is sona nanpa. (Physics is sona lon and Linguistics is sona toki.)
This is due to the fact that toki pona has a limited vocabulary; so, to form words like "physics", it simply says the phrase "science of existence" or sona lon.

vocabulary must be finite and relatively small (a few hundred or thousand words would be fine) with no synonyms

The vocabulary is just 123 words. Almost all of these are polysemantic, there's nothing like synonyms.

grammar must be fully regular

Grammar is not fully regular, but that's only because of the fact that toki pona has a few words. However, there are not that many exceptions, as in any language like English or Spanish. I have studied 12 lessons of the official book and till then, the only irregularity is the fact that li doesn't follow mi and sina.
In all, the language satisfying (1) and (2) and partly (3) to its fullest is toki pona.
Another close competitor is Esperanto. It totally satisfies (2) and (3) and partly (1). (Some semantically similar words don't seem to have common affixes in them.)
So, in all, if you are looking for such a language, go definitely for toki pona or Esperanto.
PS. Even I felt the same (unable to successfully learn natural languages due to the several irregularities) and thus myself decided to go for toki pona first (because I already know a lot about it) and then Esperanto.


Since this question was closed in Linguistics, I think it appropriate to note the answer here.

  I speak English and a reasonable amount of Finnish, and I could make absolutely no sense of any of that at all, even knowing the sound laws you posit and where the text comes from. It was basically utter gibberish from start to finish. I’m guessing the first three words are meant to be ‘with the boy’, but after that, I’m lost. –-- Janus Bahs Jacquet (9.APR.2020)


peccuvo
peccuvo (Quenya): Squirrel, lit. "nut-hider" [pec 'nut' + kuvo 'hider']
(source: Parma Eldalamberon #22 2015).


Should I include morphology in my proto language?

I am not at all certain why you would assume that protolanguages must have no morphology. PIE is just a language like any other before or since. If we review the relevant article, the Font of All Knowledge explains that morphology is the study of words, how they are formed, and their relationship to other words in the same language.

Whether your language has affixes, adpositions, or some other means of distinguishing roles all of that falls under morphology. Thus, I'd argue that it is "naturalistic" to insert morphology into your protolanguage. After all, it's already there! Else, your daughter languages most likely wouldn't have the concept at all. (And that kind of speculative invented language would be very interesting indeed!)

Standard examples, consider Latin & English:

john bit the dog
the dog bit john
the dog bit john
john bit the dog

johannem momordit canis
canem momordit johannes
canis momordit johannem
johannes momordit canem

Morphology tells us how the words are related to each other. Just because English got rid of almost all of its endings doesn't mean we did away with morphology! We just tucked it away somewhere else, in this case, word order. This is why Latin can put the agent or the patient before the verb, but English, except in unusual circumstances, can not. We expect that whatever noun comes before the verb is the agent, be it the dog or be it John.

Morphology has even been proposed for Nostratic, the protolanguage from which PIE descended.


Implement some "levels of trust" in the language: Design phrases or word choices that signal the initiated that you are talking to non-trusted people or that non-trusted people may be listening, design other phrases that signal that you can talk more freely.

I have heard (but I don't have hard references for this) that travellers and gypsies in Europe have such devices in their languages.


What are some options for alternative contrasts in demonstratives?

As the linked WALS chapter already mentions, a common distinction is to contrast near speaker/near listener/distal rather than a simple distance constrast.

One way to make such as system more "interesting" can be to add additional usages or shades of meaning to the different demonstratives, for example having one be a neutral term and the other one that is only used when specific focus on the distance is desired, or alternatively having a neutral demonstrative in addition to ones overtly marked for distance (the WALS chapter again already has examples of this, mentioning modern Hebrew and Lithuanian respectively). Korafe (TNG, Oro Province PNG) has a 3 demonstratives e, a, o in a speaker/listener/distal system which additionally serve the function of showing emontional evolvement of the speaker in utterance, with a "near listener" being the neutral default, e "near speaker" showing a great deal of involvement and o "distal" showing a desire of the speaker to distance and dissociate him/herself from the utterance1 pp.75-77.

Variables may be paradigmatically overlaid on top of a distance system, e.g. a visibility contrast as seen in Malagasy(Astronesian, Madagascar)2, or may supplement it, for example by having a 3-way proximal/distal/non-visible contrast. Other contrasts than visibility are possible, for example in Fore(TNG, Eastern Higlands Province PNG), which has a 5-way speaker/listener/proximal/medial/distal system, with proximal and distal further exhibiting a 3-way same level/above/below vertical relationship, for a total of 9 different demonstratives1.

Other than overlaying various such contrasts on a system there is also the possibility of instead having a large set of what may be called "positional stems", whith relatively specific semantics such as "up above", "on the beach" or "towards the Siberian mainland" which can then either be used as or derived into demonstratives. The various Yup'ik languages(Eskaleut, Bering Sea area) are an example of languages with a lot of these, and using various specialised and general nominal morphology allows forming demonstrative pronouns and adverbs with meanings such as "the ones up there" or "from the two entities upstairs/on the mountain", though I think there is a fair bit of variation in the actual meaning of the different stems between the different lects and langs. Section III.4 of this document covers them summarily as they are used in Gambell, St. Lawrence Island.

A somewhat similar in result but structureally different approach to this is exemplified by Nasioi(South Bougainville, Bougainville Province PNG), which rather than have a large inventory of deictic stems and a limited set of affixes instead has just one demonstrative stem a~e from which a large number of directional, as well as some nominalising and oblique affixes may derive different demonstratives such as a-un-toom-peto DEM-NOM-down.north-near.movement_toward"that down there, closer, ascending towards us from the north" or e-eʔ-dan-to DEM-INSTR-seaward-far "by going seaward far away (from us)"1.


Is Glosa more than just a relex of English?

While Glosa definitely does seem to take a lot of influence from English, it's grammar isn't quite so indistinguishable from English's as to be an obvious relex.

It avoids classic relex mistakes like including do-support, and while its tense-aspect system doesn't do anything too wild for English speakers, it uses a variety of particles rather than English's auxiliary-based periphrastic tenses. It's also noteworthy that it lacks plurals, a decidedly un-English-like feature. Both these features were likely inspired by Mandarin Chinese.

Glosa also has an article that is unspecified for definiteness, using u(n) for both "the" and "a(n)", which is a departure from its major influences -- English, which has distinct definite and indefinite articles; Mandarin Chinese, which doesn't have any such articles; and Esperanto, which possesses one explicitly definite article. However, this article's syntactic behavior does not seem notably different from the English or Esperanto articles, so it's not thinking too far outside the box there.

You could definitely argue that Glosa's departures from English are pretty small, and that it's thus still quite relex-y in spirit. It's certainly closer to normal English grammatically than Interglossa, which is reportedly based on Simple English. So it really depends exactly how far you think a conlang must depart from English grammatically to "count" as not being a relex.


How to describe a purely symbolic writing system?

I think you're overthinking this a little. While what you're considering is unusual, I can't see any reason why it wouldn't just be a written language in a logographic script.

is purely for reading and writing
has no need to be spoken and is thus not based upon letters/phonemes
is not meant to be signed, hummed, whistled, grunted, tapped or transmitted in any other way except for being printed on, and read from, a two-dimensional surface

It has no need to be, sure. But if humans or human-like beings are there they would most likely develop an oral way of "reading" it eventually.

uses symbols to represent concepts
each symbol/concept corresponds to something like an English word or sentence (e.g. 'a cat', 'the sound made by flying bees', 'the feeling that results from being beaten by a lesser foe', 'move quickly', 'why?')
symbols are highly domain-specific and exclusive — mining, carpentry, fishing, etc. would have exclusive symbols that only have meaning in their contexts

Sounds like a logographic writing system. Now it's not clear if the symbols for phrases are made up of component symbols for the things they represent or are unique singular symbols. Building symbols from other symbols would make sense, otherwise the number of symbols would grow exponentially.

symbols are arranged relative to each other (in 2D space) to communicate more complex concepts and provide context

This is on the surface the biggest difference between your proposal and human languages, which for both spoken and written language are linear.
However consider that some branches of linguistics believe that all language is representable by binary trees alone. If your language has a syntax (and if it doesn't it really couldn't be considered language) then we'd expect that it would be possible to develop a tree-based representation of it, perhaps using multiple levels of deep structure below the surface. And a non-binary tree could be converted to a binary tree.
I just spent some time looking at 2D languages, both conlangs and programming languages, and all the ones I found consisted of linear paths to follow or a tree structure. Conlangger Sai has explained their ideas for a non-linear fully 2D writing system, but they don't give an example of such a writing system. But they do say that their version of non-linearity is distinguished from linear design systems only by inelegance or convolution. I'd take this to mean that actually it is serialisable (and therefore could in theory be spoken) if not by humans then by some being of great intelligence. The essay is worth reading anyway if this topic continues to interest you.

In fact, I believe I can say that it is not possible, short of crippled or very simple specialty cases, to directly convert a linear writing to a non-linear one without either loosing a lot of meaning (NLàL), being extremely inelegant by virtue of failing to take advantage of better design (LàNL), or becoming functionally incomprehensible (e.g. the list format in which an Nth-degree array is stored in the C programming language).
So, what is non-linearity?
At its core, NL has to do with how concepts are arranged, both on physical paper and in their more abstract form. A NL system is a multigraph; its components are, or can be, extremely interconnected. There is no single traversal method, though there may be some conventional ones. There may not be a ‘traversal’ method at all, as such;


Do toki pona speakers, in practice, permit compounding more than three words?

In general, we don't want to make descriptions so long and complicated as to be difficult to follow. That is more important than exact word count, in my experience (as an admin of the "toki pona taso" group on Facebook). For example, if someone mentioned a "poki kala suli pi telo ala", I wouldn't find that at all strange as a description of an empty aquarium. However, if someone spoke of an "ilo sona lili pakala laso", I'd probably have to read it a few times to figure out what they're talking about (in this case I'm thinking of a dysfunctional blue laptop). So compounds with "pi" tend to have a bit more leeway because that groups the words together somewhat, making it easier to decipher. But shorter, simpler descriptions are generally preferred, and in practice this usually does turn out to be three words or less.


What I think you meant to ask is to create a stop consonant by stopping air flowing through the nasal cavity while the mouth is open, then releasing it only through the nose while the mouth is closed.

The problem is that having no airstream block in the mouth can mean one of two things: producing an oral vowel, such as /a/, or making no sound at all. There isn't a plosive-like sound for stopping air flowing through the nose, as there is with the mouth (in multiple places). The way you can test this is by producing a sound like /aãaãaãa/, in which you are repeatedly stopping the nasal airflow, and yet the vowel sound remains continuous because there is no stop of air in the mouth. So a stop created solely by blocking the nasal passage is impossible.

The example you give for this sound is [ĩɲ't̚?ⁿn̩], where the [?] is supposed to represent the unknown sound. This has a [t̚] with the diacritic for no release after the consonant, and a nasal release [ⁿ] after it. However, there can't possibly be anything between [t̚] and [ⁿ]. The [t̚] means that when the consonant ends, you are still blocking both oral and nasal airflow, and the [ⁿ] means that you are releasing nasal airflow while keeping oral airflow closed. There is no articulation possible in between these: if there were an oral release, the diacritic on [t̚] would have no meaning, and there could no longer be a nasal release [ⁿ] after it. The only possibility is to add another consonant, or extend the [t] as a long consonant [tː], which continues to block all airflow.

Going through your definition, nasal release seems to fit (another possibility that fits just as well is a pre-stopped nasal, if the nasal consonant is supposed to be more prominent than the stop):

  This stop is formed by stopping air from flowing into the nasal cavity with the velum

Stopping airflow is a property of a stop consonant. A stop consonant stops the airflow in the mouth as well as the airflow in the nasal cavity. (Stopping only the airflow in the nasal cavity is impossible here, as explained above.)

  while the mouth is in position to form the following consonant

Simultaneous articulation is possible, but it depends on what the following consonant is. If the following consonant is in the same place of articulation, that means there's no change.

  and then releasing it into the nasal cavity.

Releasing the stop only in the nasal cavity means producing a nasal sound. Nasal release (and nasals in general) would continue to block the airflow in the mouth, but allow it to pass through the nose.


Lojban vocabulary was deliberately constructed from scratch, in an attempt to get around James Brown's copyright on Loglan (Brown was the creator of Loglan, but continued to retain control over it. The Logical Language Group was formed to reinvent the words, beginning in the late 1980s, while retaining Loglan's grammar.

The words LLG created are the gismu, the roots that make up Lojban. They generated over 1300 of them, taking source words from Chinese, English, Hindi, Spanish, Russian and Arabic and scored many potential gismu by their similarities with the source words. The highest-scoring gismu were used; from there, the rest of the vocabulary was generated.

Now, this was similar to Brown's original process, although he used Japanese, French and German as three more reference languages, while leaving out Arabic. This algorithm was quite similar to the one used to generate Lojban's gisme. Brown's scores - and, in a sense, the LLG's - were explicitly based on learnability, which is why these languages were used. However, the implementation of the Logban algorithm resulted in a completely new lexicon, due to different weightings, quantification of similarity, and the differing source languages.

Since the relexification, the grammar of Lojban has changed in certain ways. While it would have been easier for speakers of Loglan to learn shortly after its inception, it has now diverged enough that the two languages, while based on the same principles, are now very different. I think the analogy of Anglo-Saxon and English is a good one.


Has anyone tried learning and speaking Kesh?

I have the novel, but haven't yet read it, so can't say that I've tried learning Kesh.
I used to (and may still) have the cassette tapes that came with some work about the Kesh. As I recall, it was recordings very much as you describe for the texts: songs and lore of various kinds. As I recall it was a pretty sounding language.  I think this is it.
I think it's therefore safe to say sòmeone has tried to learn it! Well enough to record it, anyway!


Can a conlang be used for machine-machine communication?

Yes, but why?

Constructed languages would be better for machine communication than natural languages because languages evolve based on how people use them but machines have some difficulties learning how to interpret those changes. With a fully constructed language, the machine can understand exactly what things mean because all of the rules and details are well document and not allowed to change through natural processes.

However, machines already communicate by sending data in specified formats; why would using a constructed language be beneficial in any way?


Is there some more detailed information about Dritok available?

This website contains a video and links to some resources. The most important is this PDF, which explains gestures ("Dritok Gestural Syntax"), phonology and grammar.


In my opinion, it reflects, if anything, more on Lovecraft's lack of linguistic understanding than anything about the language of the Old Ones, since the roots in human languages get pretty damn ridiculous, so it's really hard to imagine something "utterly unlike any found in human languages". You would have to design a phonology produced by speech organs completely unlike those of humans to approach this.

Where I to tackle this, I would go for a system not unlike Arabic, but where the root is constituted of a set of vowels instead (with maybe "consonant harmony"?).

Alternatively, Mark Rosenfelder's Kebreni has roots where vowels are swapped around a whole lot, and I doubt that metathesis as a base derivational system exists in any human language.

Generally, I'm not sure designing something literally outside the boundaries of what the human mind can handle is by definition possible, which is what Lovecraft truly was after in his poor description attempt.


Is there any research on the efficacy of intentionally designed linguistic relativity in conlangs?

To gauge how well these languages have achieved their goals, we really need to quantify what exactly achieving those goals would entail, and what steps along that path we would consider progress. What constitutes success for Toki Pona or Loglan? A world of people thinking differently? A small group of speakers who have improved the way they think substantially based on learning the language? Without knowing that, we cannot say whether these languages have achieved their goals.

Additionally, when looking for research into linguistic relativity, the place to look would be linguistic research, and we're unlikely to see much research into Toki Pona's or Loglan's influence on the minds of their speakers. Why? Well, strong Sapir-Whorf is pretty much a settled question in linguistics -- the vast majority of the field agrees it's not the case -- and weak Sapir-Whorf is more likely to be tested on natlangs for many reasons, not least of which is the number of potential subjects. Even the most popular conlangs have much smaller speaker communities compared to most natlangs, and so even if conlangs and natlangs were equal in all other ways when it came to this research (and from a linguist's perspective, they really aren't), it would be far harder to gather subjects and conduct research for the conlangs.


What are sounds that I have to include in my phonology for it to be naturalistic?

Most if not all phonemes in a language will share something with at least 2 other consonants. For example, if a language has a plosive, then it'll probably have several. Likewise, if it has a voiced plosive, it'll probably have other voiced consonants, possibly of other points of articulation.

Of course, this just the majority of the time. English, for instance has only one palatal consonant (the 'y' sound) and only one lateral (our 'l'). Though few languages have more than one lateral consonant (assuming they have any at all), and 'y' is the only common palatal consonant. All the others are rare.

There, however, a few universals. All languages have voiceless stops. They may only have one (and there are that do), but they will always have one. Also, all languages have monophthongs. So that means that there aren't any natural language out there that only have diphthongs for vowels. Note however, that diphthongs in a language can be made up of monophthongs that don't exist in the language. Take note that the English 'o' sound is actually a diphthong. English doesn't have the monophthong 'o' for some reason. This is why books that teach you foreign languages often tell you cut off the last part of the 'o' sound. What they're actually trying to get you to do is not do the off-glide that our 'o' normally has.

Beyond that though, there are no universals. No one phone is found in all languages. And exactly how many phonemes a language has varies widely. The most common is 5 vowels (the standard 5-vowel array you see everywhere) and 20 to 25 consonants. But there's languages with only half a dozen consonants, and others that have dozens. Then there's the case of Ubykh, which not only has dozens of consonants but only contains 2 vowels, which are of course prone to considerable allophony. In general, its hard to find languages that have fewer than 5 vowels, but they do exist. Several native languages up in Canada only have 4 vowels. The Semitic languages only have 3 vowels (well, modern Arabic also has 2 diphtongs, and its three monophthongs distinguish length, though classical Arabic only had 3 monophthongs, I don't know about Hebrew).


One way, while working on paper is to divide things into multiple sheets, making sure to have plenty of extra space at the bottom of dictionary sheets, or alternatively grouping things via e.g. semantic field to not run into the alphabetisation issue to quite the same extent (though this can have the issue that the semantic field of some items might not be easily determineable).

You can also use a computer program to manage your vocabulary, which offers the benefits of easy searchability and ordering. A simple spreadsheet program of your choosing is enough to work in a lot of cases, and otherwise there is the possibility of using actual vocabulary management programs. There are several free ones availible, both conlang-specific ones such as Polyglot (which also has a bunch of other functionality for organising a bunch of other conlanging-related stuff) or one developed for natlangs but still useful for conlangs, such as SIL FieldWorks (which is quite powerful but has a bit of a learning curve).


Which categories of inalienable possession have conlangs expressed?

Lojban differentiates between inalienable possession, alienable possession, and association: po'e, po, pe. But Lojban does so because its design aspired to typological completeness: it's followed the textbooks in its differentiation between alienable and inalienable possession. See http://www.lojban.org/static/publications/refgram_chunked/cllc/8/3/

  "Something is intrinsically (or inalienably) possessed by someone if the possession is part of the possessor, and cannot be changed without changing the possessor. In the case of Example 3.5, people are usually taken to intrinsically possess their arms: even if an arm is cut off, it remains the arm of that person."

So (2) physical parts. (Lojban deals with (1) as a predicate relation, not a possession.) Not (4), as far as I know. Logically (3) kinship bonds should be inalienable as well, but those too are represented in Lojban as predicate relations. (In fact the expression of relations between entities through overt predicates diminishes the role of possessives in Lojban overall.)


Are there any constructed languages designed with a constructed script, both aiming to be international and auxiliary?

I found one language called Bliss Symbolics that is

It is an International Auxiliary Language
It has its own character set, not based on Latin characters or any other character set.

I also found another IAL that doesn't use latin characters called Temenia that uses Greek characters instead of Latin.  While this language doesn't apply exactly to your question, you seem to be interested in NON-Latin based character IAL's.


Mixed writing systems - which words should have their own symbols?

There is already a conlang that kind of does that. In toki pona you can write the words in a set of pictograms (actually, there are several pictogrammatic writing systems, I'm referring here to the 'hieroglyphs' from the official book). As there are only 120 words, it's easy to have pictograms for all of them.

The only problem is what happens with other words. Names, for example. The toki pona solution is to select a hieroglyph whose word starts with the respective sound. This leaves some choice, as you typically have several pictograms available and you can choose a word whose meaning is somewhat relevant to the name. For example, Kanata (Canada) could start with the pictogram for kasi ("plant, leaf"), to link to the maple leaf; Nokisi (Norway) would start with nena ("hill, mountain"), etc.

Here you are using pictograms to spell a word, but it is easily conceivable to have a separate script for names and foreign words.

If the vocabulary of your language is substantially larger than toki pona's 120 words, you might not want to use pictograms for everything. In this case you could use them either for 'core' words (whether you define them by meaning, age,  etymology or frequency), or it could be words that have no inflections (and thus don't change, eg function words like and and for in English).

In the end it's your choice how you solve this, but there are certainly some possible justifications for different choices that you could explore.


What is the difference between an Isolating and an Analytic language?

Isolating languages do not use inflectional morphology, i.e., using affixes or other manipulations of roots to create words--all words are seperate, and none contain multiple morphemes.  Vietnamese is an isolating language.

Here's an example: the word snowman.  It has two morphemes: snow- and -man, and therefore has a morpheme-to-word ratio of 2:1.  Or take Spanish corremos, we run, which has three corr-, -e-, and -mos, for the word, present tense, and person/number.  (Please correct me if I am wrong--I don't know Spanish), so its ratio is 3:1.
In an isolating language, this ratio approaches 1:1--words stand alone and cannot be separated (hence "isolating.")  So, these concepts--"snowman" and "we are running"--would take multiple words to express, as each one would contain only a single morpheme.  Isolating languages have no inflectional morphology and no derivational morphology.

However, analytical languages are a broader category; they have a low morpheme-to-word ratio as well, but--and here's the big but--they can have a derivational morphology.  This means that while "we are running" would still be multiple single-morpheme words like in an isolating language, the word "snowman" could be a two-morpheme word.

Thus, isolating languages are actually a subset of analytic languages.  Analytic languages cover languages with no inflectional morphology and a low morpheme-to-word ratio, while isolating languages have all that and no derivational morphology.


Conlangs based on Proto-Indo-European (PIE)?

Academia Prisca published Modern Indo-European as a revival of a late stage of the Indo-European language (Northwest Indo-European, billed as the ancestor of Italo-Celtic, Germanic and Balto-Slavic). Resources published for learning include a grammar, syntax, conjugator, vocabulary, lessons and texts. Since it is meant as a modern revival, it introduces neologisms (such as read and write), but as far as I can tell, the entire lexicon is derived from cognate languages.


To answer part a), the basic syntax of the notation goes like this:

  [before] > [after] / [context]

The part after the slash gives the situations in which the sound changes occur. For example, the hypothetical spirantization of [b] to [v] intervocalically could be notated as follows:

  b > v / V__V

This rule indicates that b becomes v when surrounded by a vowel (V) on each side. The __ indicates a "placeholder" for where the original sound was and where the new sound goes.

Some other common notations are # for word boundaries and [+feature] or [-feature] to indicate the presence or lack of a certain feature respectively. For example, final-obstruent devoicing in e.g. Russian could be

  [+stop][+voiced] > [-voiced] / __#

where the __# means that the stop in question (__) is at the end of a word (#).

As for b), the best way to get an idea for what kinds of things generally happen in languages is honestly probably just by reading about lots of different languages and seeing what tends to happen. Some of the most common general processes are:

Lenition, probably one of the most common sound changes, in which a "softer" sound becomes a "louder" sound. (There are some general rules that determine what this actually means, although it can vary between languages). A few examples are intervocalic flapping in many English dialects (/bɛtər/ → [bɛɾɚ]) and the pronunciation of /b, d, g/ as fricatives or approximants, roughly [β, ð, ɣ], in most contexts in Spanish.
Assimilation is another frequent sound change. This occurs when features of a given phoneme are influenced by the phonemes around it. For example, in English and Spanish, /n/ becomes [m] before bilabials and [ŋ] before velars (e.g. <rainbow> [ɹ̠ʷejmbow], <concordar> [koŋkoɾðaɾ]).
Similar ideas are umlaut and vowel harmony, in which vowels change depending on other vowels in the word. Remnants of Germanic umlaut can be seen in English, e.g. foot/feet, and vowel harmony is prominent in e.g. Turkish, where all of the vowels in a word have to harmonize (for the most part) so that affixes have several different forms (for example, the genitive suffix is [in] after front unrounded vowels, [ɯn] after back unrounded vowels, [yn] after front rounded vowels, and [un] after back rounded vowels).

While there are general patterns, it's also important to note that some sound changes seem completely illogical and random, and just... sort of happen. The most infamous examples are the ruki rule, where [s] became [ʃ] but only after [r, w, k, j] (?), and strangest of all, Proto-Indo-European *dw to Armenian erk (???). So you really have a lot of room for creativity here, while still being able to be naturalistic.


Is there an open-source set of topics and vocabulary for making a language tutorial?

The Fundamento de Esperanto was published in 1905, so as far as I know it's in the public domain everywhere. It's one of the foundational documents of Esperanto, containing a short grammar description, a dictionary of thousands of words, and crucially a book of exercises (called the ekzercaro).
The ekzercaro fits what you're asking for. It is divided into sections, and every section after section 4 contains sample sentences and new vocabulary items. For example, this is one of the sentences in section 5:

Leono estas besto.

And these are the English definitions of the terms introduced in that sentence.

leono lion
esti be
as ending of the present tense in verbs
besto beast

Some of the designs of this course may or may not be suitable for your needs. For one thing, the ekzerzaro does not aim to have a very complete vocabulary because it is assumed that you will check the dictionary for other words you used. That means that the exercises do not teach you the word for "cat" even though they teach the word for lion very quickly.
Some sections are paragraphs to read. My favorite thing about this exercise book is that it contains a short story about a fairy interspersed throughout it. Section 27 is a discussion of Esperanto grammar written in Esperanto, which will probably have to be completely rewritten for your language.
You'll have to make adaptations depending on your language and culture. If you use these exercises to teach contemporary American English, you'll have to adapt to the ways the world has changed in the last century. For example, early on, there is is a sentence about hitting a student, and I am thankful that that is not a typical part of my country's culture anymore.


What are the reasons behind ROILA's phonology?

The process of creating the phonology is described at length in Omar Mubin's PhD thesis "ROILA: RObot Interaction LAnguage" (available from the publications page). It seems to have happened in three stages.

He determined which phonemes are most common in a number of natural and artificial languages (pp. 17-19). The most common vowels were /i, u, o, a, e/ (in IPA).
The final set of phonemes he chose included the vowels a, e, i, o and u. He doesn't indicate that the vowels were given in IPA, and indeed he later gives them the IPA values /ae (sic), ɛ, ɪ, ɑ, ʌ/ (pp. 22-23).
AA /ɑ/ was later altered to AO /ɔ/ in order to avoid confusion with AE /æ/ (p. 35-36).

The only explanation I can give for the incongruity between stage 1 and stage 2 is that he (mistakenly?) put the five vowels /a, e, i, o, u/ into his orthography, but changed their phonetic values to the English orthographic "short" vowels, which are /æ, ɛ, ɪ, ɑ, ʌ/ in American English.

The fact that he had to avoid confusion between /æ/ and /ɑ/ (in step 3), but doesn't consider any confusion between /æ/ and /ɛ/ or between /ɔ/ and /ʌ/ seems very English-centric, but which would be very strange from the perspective of Dutch, which doesn't distinguish either pair of phonemes (considering the fact he was doing his research in Holland).

Unsurprisingly, "American English speakers significantly outperformed other speakers in our setup" (p. 34). They also mention problems with Dutch accents and different regional English accents (p. 86 of this document).


While in English (and German) the days are named after (North-)Germanic gods or the sun and the moon, the months are also name after gods (and emperors), albeit from a different set (Roman), with some numerical ones thrown in (September/October/November/December); so there is no need to be consistent.

In Irish it's similar: some names have been taken from Latin, others have been added in with cultural/religious significance.

I was just looking at another language which is from a different family (Hawai'ian), but I guess they no longer have the weekdays they had before they had contact with Western powers: their days are literally "Night-one" (Po'akahi) to "Night-six" (Po'aono), plus Sunday being "Pray Day" (La pule). The month names are just localised versions of the English ones ("Ianuali", "Pepeluali", etc). Indonesian weekday names also seem to be derived from the names of (arabic) numerals.

You could choose anything where you've got a set: celebrities/rulers, planets, plants, animals, whatever. I don't think anyone would look at it and think it weird to have a "fox-day", or an "oak-day". Add a regular prefix or suffix to the word that means something like "day", and it should look alright.

And if you have two half-weeks, why not have an animal half followed by a tree-half?


Conlang where the initial consonants of words have meaning?

Many languages mark things like those you mention at the beginning of a word. Noun class in Swahili and the other Bantu languages comes to mind. It doesn't require all roots to begin with a vowel; the prefixes also include vowels (as in Kiswahili).

Russian has prepositions (not prefixes) such as в, which are pronounced as part of the onset of the following word (and it does sometimes cause initial consonant clusters), in addition to case. There's no reason why a conlang couldn't have the same as a prefix for gender, case or tense.

You might also want to look at the WALS chapters Prefixing vs. Suffixing in Inflectional Morphology, Position of Case Affixes, and Position of Tense-Aspect Affixes for lists of languages that have these features as prefixes.


The book by jan Pije and jan Lope says:

We've learned how to address people and how to make commands; now let's put these two concepts together. Suppose you want to address someone and tell them to do something. Notice how one of the o's got dropped, as did the comma.
jan San o, ... - John, ...
... o tawa tomo sina! - ... go to your house!
jan San o tawa tomo sina! - John, go to your house!


What reasons would there be for not having a human conlang with only vowels?

A vowel-only language is surely constructable (and I think, learnable, too), but I am afraid that it will be instable against evolutionary pressure.

Vowel sequences like /aua/ or /aia/ tend to develop into glides /awa/ and /aja/ giving raise to the first consonants in the language, and at the hiat between two vowel a third consonant, the glottal stop, may materialise.

Efficiency of the coding is also a concern, maybe tones can take over the role of consonants in creating more different syllables from the vowels.


How much Dothraki was created prior to "Game of Thrones"?

The majority of Dothraki was indeed created for the show.
George has stated the he is not quite a linguist in that sense (comparing himself against Tolkien).

I don't have a whole imaginary language in my desk here, the way Tolkien did.
Tolkien was a philologist, and an Oxford don, and could spend decades laboriously inventing Elvish in all its detail. I, alas, am only a hardworking SF and fantasy novel, and I don't have his gift for languages. That is to say, I have not actually created a Valyrian language. The best I could do was try to sketch in each of the chief tongues of my imaginary world in broad strokes, and give them each their characteristic sounds and spelling.
 So Spake Martin, YET MORE QUESTIONS, July 22, 2001

He mostly just makes words as needed.

"DO YOU HAVE THE LANGUAGES ALL PLANNED OUT?"
No he'll make up a word here and there but he does NOT have the whole language written.
 So Spake Martin, BOSKONE (BOSTON, MA), February 14,2003

Even going as far to say that now he will have refer to the creator of the language for the show in his future writing.

Of course, he then added that with HBO having created Dothraki through the work of David J. Peterson, he feels like now if he wants to have Dothraki (and Valyrian as well) he'll have to refer to Peterson's work to get it "right", or ask Peterson himself how to say something in Dothraki.
 So Spake Martin, STOCKHOLM AND ARCHIPELACON REPORT, June 28, 2015

The show selected David J. Peterson, an expert language creator from the Language Creation Society to create Dothraki for Game of Thrones

Peterson drew inspiration from George R.R. Martin’s description of the language, as well as from such languages as Russian, Turkish, Estonian, Inuktitut and Swahili. However, the Dothraki language is no mere hodgepodge, babble or pidgin. It has its own unique sound, an extensive vocabulary of more than 1,800 words and a complex grammatical structure.

Which as of September 21, 2011 has over 3,000 words

Also, for the curious, Dothraki is now up to 3,163 words: More words than Mr. Padre Tony Gwynn has hits, but still a ways to go to catch Pete Rose (and my guess is Dothraki will have double his number before he gets even a sniff of the Hall of Fame).


What are some strategies for setting verbs apart from other words without knowing it's a verb

In Lojban, cmevla (proper nouns, like .alis.) are the only words ending in a consonant. Depending on how big your vowel inventory is, you could make all verbs end in a vowel and everything else end in a consonant. That way, they are easy to pick out and still provide much variety.

To generalize that, you could just take the last sound of a verb from any distinct group of sounds, like nasals, fricatives or labiliased consonants ("labialiseds"?). One could also use that to make word correspondences, e.g. nominalization changing the verb's final sound to the corresponding nasal.

You also mentioned position in the sentence. Having the verb in a specific position (first, last, second, etc.) is a good way to distinguish them, but depending on your language, dependent clauses could get difficult.
Of course, coupling position in sentence with a specific ending sound provides a nice way to recognize sentence borders. There, one could again use the ending sound kind to modify the verb to signify a dependent clause.


Do constructed languages need to have their own letters, words, & sounds?

No, a conlang does not need any of it. You can construct a language based on gestures (a sign language) or on whistles, or on other signals transporting information. You can construct a Pasigraphy, i.e., a written-only language.

But for a more conventional design that meets our intuition of language, you will probably use sounds and words, and letters to write the language. It is up to your creativity to come up with original sounds and letters, but there is definetely no need to do so. Many conlang designs rely on sounds (and complete phonologies) that occur in natural languages, and use usual letters (mostly Latin, but also Cyrillic or other existing writing systems like Runes) for writing. The words of a conlang are most often original but I even can imagine a conlang without original words: Draw existing words for natural languages in an eclectic way or create a conlang closely based on a natural language or a group of natural languages. The vocabulary of IALA Interlingua is hardly original.


List of conlangs whose goal is to minimize ambiguity

Here are the top five best-known (see footnote 1).

Lojban at lojban.org is the better-known successor of Loglan.
Loglan at loglan.org is based on formal logic and is to some extent not unlike a transcription of formal logic (see footnote 2).
Ithkuil at ithkuil.net is like the two above but is as compact as possible, allowing to read, write, say, and hear complex thoughts and ideas in a matter of seconds.
Ceqli at http://ceqli.pbworks.com/w/page/5455970/FrontPage is originally inspired by Loglan but later a bit more naturalist
CycL at http://www.cyc.com/documentation/ontologists-handbook/cyc-basics/syntax-cycl/ is a lot like a programming language intended mainly for neural networks. It's more suitable for expressing knowledge than for communication.

All these have Wikipedia articles under their name, except Ceqli (available on Dutch Wikipedia) to which you can go via the link given.

Footnotes: 1) These languages (except Lojban and possibly Ithkuil) don't actively disambiguate phonemic ambiguity. That's the difference between 'an app' and 'a nap'. It resulted in English a nickname, from an ekename.
2) An example of formal logic, using prerequisite axioms [PERSON(a) 'A is a person', HAVE_ALIENABLE(a, b) 'A has B alienably', HEAD(a) 'A is a head'], is ∀ x: PERSON(x) → (∃ y: HAVE_ALIENABLE(x, HEAD(y))) ('Everyone has a head', 'For every X, if X is a person, there exists some Y which X has alienably and which is a head').


good naturalistic system for Germanic language?

A long time ago, I was involved in a project named Folkspraak to create a Germanic conlang. It was entertaining, but it did not went very far at that attempt.
Some grammatical hallmarks of Germanic languages are:

A very simple tense system, present and past only, all other tenses are periphrastic and later acquisitions
A past participle as third principal part of the verb
Weak verbs ("regular" verbs) and strong verbs (looking irregular, but in fact they are regular, too, falling into several classes of strong verbs)
Related pairs of strong and weak verbs like fall/fell where the weak verb is a causative of the strong one one (to make fall). More examples for this relation from German trinken (strong verb, "to drink")/tränken (weak verb, "to water (animals)"); springen (strong verb, "to jump, to leap")/sprengen (weak verb, "to explode; to water (lawn); to gallop"). A notorious one is erschrecken (strong verb; "to get frightened, to startle)/erschrecken (weak verb; "to startle so., to frighten")—notorious because the infinitive and some other form coincide and there is some confusion and a tendency to use sich erschrecken (weak verb) instead of the original strong verb in Modern colloquial German.
Modal verbs that are special (preterite-present)

For nouns there is less common ground for the Germanic languages, while German retained four cases and three genders for nouns, those features are partially or completely lost in some other Germanic languages. You can do whatever you want (strive for simplicity or keep a more complicated system at your wish). Same for adjectives.


It is obviously not completely dead, there is at least one blogger posting short news items in Latino Sine Flexione here http://nuntios.blogspot.com/search/label/Latino%20sine%20Flexione

I don't know whether there is a functional speech community for this language left.

EDIT: There is another life sign of Latino sine flexione: Someone created a LaTeX package for it!


Is there any importance of "dots" while constructing alphabets for a language?

In English, the dot does not carry meaning. It's just part of the lower case letters i and j. There is no dotless base form. Note that the letters i and j aren't dotted in every font or variant of the Latin alphabet. Notably, there is no dot in Gaelic type.

Turkish, on the other hand, does distinguish between dotless I/ı (representing the phoneme /ɯ/) and dotted İ/i (/i/).

Historically, the tittle was added to improve legibility. In late medieval blackletter, the letters i, m, n, and u were basically all made up of vertical strokes, minimum basically was ııııııııııııııı. So a dot was added to i: ıııiııiıııııııı.

Dots can be used for a variety of purposes: Inuktitut syllabics (a constructed script for a natural language) use dots to indicate vowel length, while the Hebrew abjad uses them as vowel marks (niqqud), so does Tengwar. In Arabic, they are used to distinguish letters that have come to otherwise look identical (i‘jām).

Dots are an easy way to derive one symbol from another.

As jknappen has already said: it's up to you what you want to use dots for in your script or if you want to use them at all.

A thing you might want to consider is the medium your script is supposed to be written on. Dots can be easily produced with ink on paper but they are harder to carve in wood where you might want to use a short line instead. Of course, if it is meant to be mainly printed or displayed by screens, reproducibility is much less of a concern.


How does one write an easily decipherable language?

It may be impossible.
All successful real-world language decipherments—Linear B, Egyptian, Hittite—have involved connections with other known languages. Linear B (Mycenaean) is closely related to Classical Greek, for example; Egyptian is related to Coptic and a bit more distantly to Hebrew and Arabic; Hittite is a distant relative to the whole Indo-European family.
Even if you only look at the writing system, figuring out Linear B involved connecting known place names like "Knossos" with words that were more common in inscriptions at Knossos than elsewhere. Ventris's biggest breakthrough came when he hypothesized that Kober's ??-C₄V₂-C₂V₂ might be k-no-so, which gave tentative values for C₂, C₄, and V₂, and extrapolated from there.
With an alien language, we have no idea what names they used for places. We don't have any loanwords or names transcribed in other languages to compare against. We don't know if they use anything resembling phonetics to communicate—or if their language is even fundamentally based on recursion like ours is.
But…
Maybe humans and Thulians have never made contact before, but the Borean language is known, and an ancient Borean imperial decree—translated into Thulian—is found in the excavations.
Maybe Thulian and Borean are part of a language family—that hints to the linguists that Thulian might use a stack-based syntax (or whatever other weirdly inhuman feature you like) just like Borean does.
Maybe there's some old Thulian technology stored in Area 51, taken from a crashed spaceship; it contains records of an early Thulian attempt to figure out human languages (maybe even a rudimentary grammar of English written in Thulian).
Maybe they can find a live Thulian, or, failing that, an artificial intelligence of some sort, which can give them real-time feedback—a native speaker is orders of magnitude more useful than any inscriptions can ever be.
Maybe Thulian is related to some obscure Earth language(!). There used to be contact between Earth and Thule, which stopped millennia ago, and Thulian shares a distant ancestor of Etruscan/Sumerian/Pirahã/your favorite real-world language isolate. (Though be prepared for a lot of strong linguistic opinions if you decide to weigh in on the Pirahã controversy!)
Maybe universal grammar (in whatever form you like) is universal even across planets, and Thulian still has recognizable levels of phonology, morphology, and syntax.
None of these on their own would make decipherment easy—but the more you add, the easier it gets. At some point, it becomes possible, and that's what matters for a story.


Which character is best to use for a schwa? Ь or Ъ?

If you're going to use one of <ъ> or <ь> to represent /ə/, it's probably better to use <ъ>. This letter is currently used to write /ɤ/ in modern Bulgarian, whereas <ь>, as far as I'm aware, is only used to write actual vowels in dead Slavic languages. Moreover, because <ь> historically represented a short front vowel, it is used in modern languages like Russian to indicate palatalisation, something that does not accompany /ə/, whereas, e.g. in both modern (and more widely in pre-reform) Russian, <ъ> marks not a vowel but the absence of palatalisation.


You are likely having a hard time reading these tengwar because of three big problems:

the style itself is such that the tengwar are slanted and written
calligraphically
the mode used to write them is unusual and not common for Elvish languages
there are no spaces between words!

The page linked should provide assistance; the issue is that the tengwar and tehtar behave (like in several modes) as in an abugida: the vowels are diacritics to the "host" consonants, and a null consonant is used for vowel-initial words (even when the words aren't separated by spaces).


Is there an existing constructed language that once was dead?

Since conlangs by their very nature don't start out with any speakers, and in most cases the author is not immediately a skilled user of the language, conlangs tend start out dead, and so under this definition almost any conlang with a speaker community would technically once have been dead.

However, going by "dying" as having a community, then losing it, reliable data is somewhat hard to find, as a small remainder of a community (which could be as little as two individuals' private postal correspondence) can be very hard to locate especially in pre-internet days, though it seems that Idiom Neutral, published in 1902, once had largely fallen out of use, but has recieved a recent revival by enthusiasts in a couple of internet usergroups and blogs.


What notation do constructed sign languages use?

There are very few constructed sign languages. On the subreddit, we sometimes get questions about whether anyone has made any, which usually don’t get many replies. This one here has some examples given, and also links a website which has basically the answer to your actual question:

SLIPA is an attempt by David Peterson to fill the lack of sign language transcription systems. However, due to it being ASCII-compliant, it ends up rather unweildy and I’ve never seen it employed in actuality. Nevertheless, the website provides important information that could be useful for you to derive your own system tailored to your language, and to use as a reference for comparing your own notation to.


How could a syllabary be adapted for a language with a complex syllable structure?

Linear B is an interesting example for this:

This system was apparently designed for a non-Greek language, as it did not fit the sounds of Greek very well. In fact, it is likely that Linear A was used to write the pre-Greek language of Crete, and the incoming Greeks adopted this writing system for their own use, but without changing how the system fundamentally works. In doing so, they developed "spelling conventions" to represent sound patterns found in Greek but not in the syllabary.
First, there are many Greek sounds that are missing in Linear B signs, such as [g], [kh], [gw], [b], [ph], [th], and [l]. To solve this problem, signs for similar sounds are used instead: p-signs are used for [p], [b], and [ph]; k-signs are used for [k], [g], and [kh]; t-signs are used for [t] and [th]; q-signs are used for [kw] and [gw]; and r-signs are used for [r] and [l]. However, while this convention was likely easily understood by ancient Mycenaean scribes, it took modern scholars a lot of theoretical analysis and work, plus comparison with later Greek dialects and reconstructed Mycenaean words to rediscover how this system works

And more relevantly:

Another inadequacy stems from the fact that Linear B signs usually represent Consonant-Vowel (CV) syllables, but the syllabic structure of Greek allows initial consonant clusters, ending consonants, and dipthongs. In the case of a syllable with a initial consonant cluster, individual consonants in the cluster are written by a CV sign whose vowel matches the vowel of the syllable. Therefore, for example, the word tri is written as ti-ri, and khrusos as ku-ru-so. In the case of ending consonant, the situation becomes more complicated. Ending consonants such as [l], [m], [n], [r], and [s] are not usually written, whereas other consonants such as [k] and [p] are written in a way similar to initial consonants.

So one way to represent consonant clusters in a syllabary is to pick a "dummy vowel" that gets read over when the word is pronounced, as in Linear B, or Japanese katakana when writing foreign words.
The question doesn't mention just how complex the syllable structure is; if it's not too complex, you can also do what Japanese hiragana does with the ん character and have a single character that represents a syllable-ending consonant. Or something like what Brahmi does for vowels and nasals, adding strokes and dots to represent them. Maybe if kra, tra, and pra are common syllables in your language, you could have a stroke that represents an r between the consonant and vowel, and add it to the characters for ka, ta, and pa.


Has any nation ever employed a constructed language in the military as code talkers?

The Israeli military uses a relexification of Hebrew called "NADBAR", according to Conlang Wikibooks†. A relexification of a language remains the grammar (by definition) and writing system (in practice) but gains new words. Your first (semi-)conlang was probably a relexification of English or another language.

Many languages used in the military are not immediately conlangs; rather, they fall into the dome of cryptology and cryptography. Quote:

  Some use the terms cryptography and cryptology interchangeably in English, while others (including US military practice generally) use cryptography to refer specifically to the use and practice of cryptographic techniques and cryptology to refer to the combined study of cryptography and cryptanalysis.

The military also uses (forms of) signed languages. Here are some common signals:

Common army signals (link instead of image, for image is too big and distracting)

†Note: (from comment by b a):

  According to Hebrew Wikipedia, "Contrary to the popular notion, the goal of Nadbar is not concealing information" (so not really cryptography) See the corresponding English Wikipedia page for its use in the US military.


I agree with thrig that you would need to create something to handle this case. toki pona doesn't normally differentiate between specific sides, and this is one of those cases - there are no ways to describe "left" and "right" natively.

However, I'm not sure thrig's examples necessarily do the job the way it needs to be done - poka wawa could work, but it implies something about strength, when that's not often what people mean by dominant and non-dominant side. And meli/mije are not used to describe strength in this way.

Instead, I would suggest using:

poka lawa: your leading side.
poka kama: your following side.

No matter what, though, if you use this you're going to have to indicate that you're setting it up as such in advance. You can still do this in toki pona itself: mi toki e ni: poka wan li poka lawa. poka tu li poka kama. This sets up that you're saying one side is your poka lawa and the other side is your poka kama.


Based off the audio file from this website, and the audio files from this website, we can infer that Toki Pona is a "fast" language. It uses 6 - 8 syllables / second, except with punctuation, where you delay for the same time as English.

Some Youtube videos stick to the contrary, but their speed is slow to make it easier for nonexperienced Toki Pona speakers.


Is there a practical lower limit to the number of morphemes required in a language?

Outside of Toki Pona, there haven't been any "majorly" successful attempts at making an oligomorphemic language — as far as I know. While derivational and inflectional morphology could be done away with (and its information load transferred to syntax I guess), the question is how many semantic morphemes is too few before a language becomes incomprehensibly vague.

I'm guessing the theoretical lowest lower limit is the set of 63 semantic primes of the Natural Semantic Metalanguage (see also Goddard's Oxford Handbook of Linguistic Analysis, 2010). Wierzbicka and Goddard propose that you could theoretically explain almost all necessary concepts by building up from this set of primes, and lacking ways to express some of them will leave you incapable of expressing at least a subset of all possible topics.


Are there speech communities for Tolkien's Elvish languages?

It is not possible to speak Tolkien's Elvish Languages.

This may confuse some people, considering how much nonsense there is online for "how to speak Tolkien's Elvish" (there's even a wikihow article about it). But according to the Elvish Linguistic Fellowship, an organization dedicated to the study of Tolkien's invented languages, even Quenya and Sindarin are simply not learnable:

  The vocabulary, grammar, and syntax of Tolkien's invented languages, even of Quenya and Sindarin, are far too incomplete to allow its casual, conversational, or quotidian use. As Tolkien himself stated, "It should be obvious that if it is possible to compose fragments of verse in Quenya and Sindarin, those languages (and their relations one to another) must have reached a fairly high degree of organization — though of course, far from completeness, either in vocabulary, or in idiom" (Letters p. 380). Indeed, it was never Tolkien's intent to make Quenya, Sindarin, or any of his languages into spoken, written, auxiliary, or otherwise "useful" forms; rather, they were done for purely personal enjoyment. As Tolkien wrote, "It must be emphasized that this process of invention was/is a private enterprise undertaken to give pleasure to myself by giving expression to my personal linguistic 'aesthetic' or taste and its fluctuations" (ibid.)

-- the aforementioned E.L.F.'s FAQ, emphasis mine

An article entitled "Elvish As She Is Spoke" goes into more depth about the history of Tolkien's Elvish and the problems with trying to learn to speak it.

"But don't they speak Elvish in the films?"

The lines and lyrics from Tolkien's languages in the Peter Jackson films were written by David Salo, a linguist who wrote a grammar of Sindarin. He built the dialogue he needed on the limited available attested vocabulary, coining some new words himself but with the goal of making it intelligible to any viewers familiar with Tolkien's languages. To quote him:

  Why is there Elvish in the movie? Why did Peter Jackson care enough to strive for some accuracy in the way language is presented? (...) The Elvish in the movie is addressed to the minority of viewers who know something about the languages. And what are they going to want to do when they hear the Elvish sentences?  They're going to want to figure out what they mean, and why they mean what they mean. Part of my intention, my particular vision and contribution to this movie, was to create sentences which would be intelligible to the people who study the languages (...) I'm enormously happy to see some people saying based on their knowledge of Elvish, great or small, that they recognized and understood some of what they heard on the screen. That's great - that's exactly the kind of effect that I was looking for.

Here is a collection of the translated dialogue from the LoTR films, with the attested and unattested vocabulary in each line listed. As you can see, a lot of it is attested in Tolkien's works, but there is quite a bit that needed to be invented for the film by Salo. There have been other projects based on Tolkien's Elvish languages in the past as well, but the real fanatics tend to view these as "pseudo-Elvish", since anything not invented by Tolkien can't really be said to be part of Tolkien's Elvish. That said, there's nothing wrong with seeking out resources for and learning these expansions on Tolkien's conlangs -- just know that calling any one an authentic recreation of Tolkien's Elvish is pretty flawed.

Tl;dr

  Since Tolkien never fixed his languages firmly or described them completely enough to provide any such comprehensive and corrective model (that never being his goal), ... it is consequently a further inescapable fact that no one has or ever will be able to speak Quenya and Sindarin, any more than anyone will ever (again) be able to speak, say, Etruscan or any other fragmentarily-attested non-living language.

Look into the E.L.F.'s resources (though they do seem a tad outdated) if you're interested in further exploring what information we have about Tolkien's Elvish.


By what means might the roots "let" and "ly" mean the same thing in a naturalistic conlang?

It is a two-step process, and both steps are very natural and frequently encountered in natural languages. The steps may occur in the other order as well, but the order here deems more common to me.

Loss of the final stop let -> le. This occurs very often, French is a prominent example of this because the final stops are preserved in writing, but lost in pronunciation (with some exceptions when there is a following vowel).
A vowel shift le -> li, in this case a raising. Again, this step is very natural because /e/ and /i/ are neighbouring vowels in the vowel diagram.

These two steps use your assumption that let is the older form that evolves into ly. A more sophisticated construction could assume a common ancestor for both forms, e.g., lit that evolves to li by the loss of the final consonant and to let by lowering the vowel.

EDIT: As you are generally interested in sound laws, there is a resource named Index Diachronica that lists a lot of sound changes collected from many sources.


Can Root Words Be Derived From Other Root Words

Sure. And like always, real languages got there first.

In Quechua, for instance, fist person singular ("I/me") is Ñuqa. 1st person plural inclusive (we, all of us) is Ñuqanchik, and exclusive (us, but not you) is Ñuqayku.

Second person singular is Qam, plural Qamkuna. Third person singular is Pay, third person plural Paykuna. -kuna is simply the Quechuan plural suffix. "House" is wasi, "houses" wasikuna.

"You, the person I'm talking to" Qam, "You, the people I'm talking to" Qamkuna.

It's quite common in many languages that "I" and "you" are distinct; after all, I'm usually the most most important person in the universe, and you just live there. Third person pronouns seem to often be derived from pointing words: "that thing over there" becomes "that person over there".


Soudwegian: A Swedish-German process similar to what happened to English and French?

Yes, that is a realistic choice in fictive works.

Abstractly
Nobody can be 100% sure of how realistic it is, and in conlanging—or even fictive worldbuilding in general—you may always be a bit unrealistic. Think about some fictive works or conlangs you know placed in another world; they make many assumptions that extraterrestrial life has two eyes, hair on their head, ~1m–2m tall, and their mouths have lips, teeth, an alveolar ridge, a glottis and a tongue. (Your idea takes place in our (but alternative) world, but the point of the example still stands.)

Linguistically
There could be many ways of forming such a language, of which the most realistic are (chiefly from Mark Rosenfelder's Advanced Language Construction):

A creole. A creole is, by definition, a pidgin which has native speakers. A pidgin is formed when two languages try to communicate, for reasons of trade or otherwise. If you read about Tok Pisin (etymologically "talk pidgin", a creole), you'll see a lot of simplified grammar (and, as you say, English is already quite simplified).
From Swedish, but developing independently from main Swedish. As a native English speaker born in Holland, informal English I use to talk to my family is very different from main English. For example, I'd say *How late is it? (from Dutch Hoe laat is het?) by accident. Also, lots of sound changes happen. Even though I don't merge /w/ and /ʍ/, many people I know that used to be able to can't anymore, and my rhotic in both Dutch and English is [ʁ], even though neither Dutch nor the Scottish dialect of English has it.
A combination of the two. After a period of isolation (independent development), the Swedes start talking more like the Germans (like creoles, but slightly different)—a combination of the two.

So, even though it might be somewhat unrealistic, within the dome of fiction it isn't unrealistic enough to stop you.


What are the places where I can attach an indication of tense in relation to a conjugated verb action?

Since it’s your conlang, you can put it wherever you think is best—but my inclination would be immediately adjacent to the verb it applies to. Your sentence structure seems to be VSO, so my initial inclination is to place it immediately after the verb.

For your sample sentence, Sepyew sonsato eu bítõe de è roum de aqua.

For the translation of a sentence like "The truck carried the food Joe ate", you would need two tense indicators, one for each verb: "it-carry past-tense the truck [he-eat past-tense Joe the food] sentence-as-object-indicator". If only one verb is in a non-present tense (The truck carries the food Joe ate), you apply the tense indicator to the verb that is in the non-present tense:"it-carry  the truck [he-eat past-tense Joe the food] sentence-as-object-indicator".


Are there constructed languages with a tense system inspired by Theory of Relativity?

I think you are looking for the constructed language called Lojban
According to an earlier reference to its grammar,

Technical note for readers conversant with relativity theory: The Lojban time tenses reflect time as seen by the speaker, who is assumed to be a ``point-like observer'' in the relativistic sense: they do not say anything about physical relationships of relativistic interval, still less about implicit causality. The nature of tense is not only subjective but also observer-based.

Here is a specific reference to its use of a "relativistic tense"

There's been a proposal to have words expressing relativistic tenses that are different > from usual past, current and future used in most historical natural languages.
How this 4th tense would work? Consider we are 3 light years away from a star that explodes. And in 3 years we see the explosion. So what? How do we use this 4th tense?
It would be a tag with a modifier ue'ei.
ue'ei = [MOhI] 4th relativistic tense modifier

There's also an interesting set of charts describing in detail the use of relativity theory in Lojban on the current lojban language site.

Lastly, in the Lojban Reference Grammar site, there is this little intriguing tidbit on Lojban tense usage.

In Lojban, the concept of tense extends to every selbri, not merely the verb-like ones. In addition, tense structures provide information about location in space as well as in time. All tense information is optional in Lojban