Another occasionally used criterion for discriminating dialects from languages is that of linguistic authority, a more sociolinguistic notion. According to this definition, two varieties are considered dialects of the same language if (under at least some circumstances) they would defer to the same authority regarding some questions about their language. For instance, to learn the name of a new invention, or an obscure foreign species of plant, speakers of Bavarian German and East Franconian German might each consult a German dictionary or ask a German-speaking expert in the subject. By way of contrast, although Yiddish is classified by linguists as a language in the "Middle High German" group of languages, a Yiddish speaker would not consult a German dictionary to determine the word to use in such a case.
Why is Yiddish not a dialect of German?
a Yiddish speaker would not consult a German dictionary