Back around 1994, I had a CD version of "The Secret of Monkey Island" that contained five language versions of the game. I brought it in to my high school French class and my teacher LOVED it. Thanks to that, I can still say, "How appropriate: you fight like a cow!" in French. (Didn't come in so useful at that job where we actually communicated with Francophone schools and businesses, though...)
Similarly, I've played my MMOG of choice -- EQ2 -- on the French servers from time to time. I haven't worked up to having the gall to group with anyone yet but it's still interesting to be in that kind of informal environment in my second language.
That said, even after two years of college Chinese I don't think I'd ever be bold enough to attempt to do that with Mandarin! But I like the idea of an MMO meant for learning. I'll have to give it a try.