I know I mentioned it before, but I can't help think that the existence of pure, unchanging, preexisting, absolute languages out there in the cosmos... Ones that people can be born with knowledge of or suddenly know fully and completely after a magic accident or the like... Greatly influences the stability, uniformity, and availability of linguistics and literacy. Heck, every gutteral pre-literate society is one stray lightning bolt away from a storm-sorcerer bringing them a fully formed language and alphabet.

In fact, I'd be somewhat surprised if nearly all languages spoken anywhere except the most extremely isolated (in the planar sense) places can't trace their origin back to one of those primal languages... Elven from Sylvan (with a touch of Celestial); Dwarven from Giantish which is from Primordial; Common using almost unrecognizably degraded draconic phenomena; etc.