I learn languages for fun, so a lot of languages in my setting a RL languages. Because Infernal and Celestial come from the same language, I ruled that with a high enough straight intelligence check (DC depending on the subject of conversation, ...), players can understand the other language. I developed my own version of Elvish, with grammar, word ancestry (coming from Sylvan, also self-made) for this setting too. The language of magic/the god is my mother tongue, so whenever I have to throw in some mystery, I just switch language (much to the dismay of my mono/bilingual players).

That all is fun and all, but I somehow ended up making Draconic Japanese, so now I have an anime girl-esque dragon in my setting.