I found an interesting post in Ben Scerri's blog
versamus ; we roll: Conlanging for Fiction – Part 2 – In Creative Writing.
One of my conlangs, Khangaþyagon is part of the background of a fantasy book I've been writing on and off (more off than on) for a few years. I wanted the magic in the book to be believable, so I thought that the spells should be in an ancient language, now known only to wizards. I wanted the incantations to be meaningful, on the assumption that readers, when coming across a spell, would be able to spot gibberish, and that that would reduce their suspension of disbelief. There's one famous fantasy series that I lost interest in partly because I couldn't suspend belief in its magic system - which was one of the things that provoked me to think "I can do better than this". I'm planning to have an appendix at the back of the book translating the spells. One thing I'd note is that it's OK to have a glossary at the back of the book, but it the reader has to consult it three times by the end of the first page, you're (a) getting a bit carried away, and (b) Frank Herbert.
The other thing I've done is that there's no "Common Tongue" in my fictional world. Wizards can learn languages supernaturally fast, but they still have to learn. My protagonist isn't a wizard (yet), and for much of the book is dependent on somebody else to translate for him. This puts him in a vulnerable position. At one point, I've also got a scene where one character tries to teach him her language. In another point, an old woman who learnt his language in her childhood mentions that she can never remember when to use a certain prefix in it, and he gets hopelessly tangled up trying to explain it. I'm trying to give an idea of the linguistic richness of the world, without the reader getting bogged down in fine details.
I'll be following versamus.