To provide a little background, I'm writing a novel set in a fantasy world, within which there are multiple languages.
I'm applying the rule that any speech the viewpoint character (I'm using subjective third person, by the way) is able to immediately understand is rendered as English, and everything else is written in the original language (as he would hear it).
The trouble is that some characters speak a language that is very similar to the viewpoint character's own language. It's not close enough that he'd understand every word, but he could probably get a rough idea of what they mean.
My question is, how can I convey this (in the sense of putting the reader in the character's head) as clearly and simply as possible?