Unfortunately, the game would need to be new, at least in terms of the story/quest content. Think about how Rosetta Stone works. The clue is "gato" and you clic

Unfortunately, the game would need to be new, at least in terms of the story/quest content. Think about how Rosetta Stone works. The clue is "gato" and you click the picture of a dog from 4 choices. The program gives you a big fat red X, and from now on you, you know that "gato" does not mean "dog". It's a very deliberate, clever learning process that really works. The problem with Rosetta Stone is that it's not fun, there's no end motivation and people always quit. That's where the addictive motivational aspects of an MMO would come in. So each of the quests would need to be tuned to this kind of immersive trial/error learning (which is how we acquire new languages). Right now, if my memory serves me, most Warcraft quests don't even require you to read the text or listen to anyone speaking. You just follow a nav arrow or go towards a dot on the mini map. Simply switching languages would not get the job done.