Familiars teach languages
Some familiars speak different languages and you can have them speak in that language by possessing them or using the -a command. Your PC will even make a lore check to understand them, but it does not activate -language learning. Can we get that changed? It makes sense that your pixie familiar could teach you or others Sylvan. That Mephits could teach their language, etc...
Obviously animals can't teach animal languages since that's unlearnable in game and only granted to certain classes.