I used to play a game in which 75% of the population was Singaporean (sp?)
Turns out the only things I really learned of their language was mostly just cursing, sarcasm, and trash talk.

The experiance was mirrored when I played TFC for a German clan one year. I was bored, had some free time after school which was right as their matches started, and I had heard that they had some really skilled players. Again, I learned most of the trash talk, and curse words that normally go along with competition.
To this day I still remember how to say:
I've got the flag. / They have a sentry gun up. / Security is down. / Good game.
So after playing with them for 6 months, I could effectivily understand and communicate things needed for match play without thinking about it. However, none of that really translates to real life usage.

So if you do try and use WoW to learn the language, you will have to actively seek out conversations with people about their lives and whatnot. If you don't, you'll only learn whats commonly used in WoW. Looking for Group etc.