The simple solution, as in all RPG is a language called common. This language has been around in nearly all RPGs(CRPGs and PnP) and can be spoken be nearly every one from Highest Elf to the Lowest Goblin. No one knows were it came from but it might have originated to make the life of the Game designer a lot easier.
If you do get stuck you could probly make up some magic iteam or little fish to explane it away.