I'd agree with pretty much what's already been said. Write it in the language you use -- I'm presuming English, but it may not be.So I’m writing a medieval fantasy novel. The problem is, English doesn’t exist in my world and I’m afraid that it will break the immersion if my characters talk to each other in English. Do I have to invent a new language? Please help, I can’t start writing until I figure it out.
I've read Chaucer's The Canterbury Tales in (14th C.) Middle English, and attempted to read Beowulf in (8th - 10th C.) Early English, and both are so different from Modern English that they're effectively unintelligible to many modern readers without translation. Just try to avoid any anachronisms.
The other problem with inventing a language is: what if your story was really good, got traction, and ended up translated into another language? The translators would have to deal not only with English to Romanian (for example), but also "Inventish" backtranslated to English and then to Romanian.
If you need a character who comes into the story, but doesn't speak the language of the land well, you can always write it as either broken English, or you can have them say something your MC doesn't catch, and another character translates. Alternatively, misunderstandings can produce the most comical, and tragic, of results. And, there are always hand gestures and pointing, things I've had to rely on in various countries.
Most of all, just write the story first, to get an idea of what you want it to be, and to say. And, best of luck with it all!