Apr 5th 2007 9:50PM We once had a non-native English speaker in our guild, and it wasn't that big of a deal. Every so often he would ask a question or make a statement that had us all scratching our heads, but our confusion was always quickly cleared up by asking for clarification. If we ever have another ESL guildie, I doubt it would bother me.
As for learning English by playing WoW, I think it's a terrible idea. First of all, how often do people use complete and proper sentences with correct grammar and spelling? Practically never. Second, the vocabulary used while playing WoW is pretty useless and irrelevant in real life, and the names for some things would be misleading for someone wanting to learn the language.
I think that if there was a MMO designed specifically with the idea of learning a foreign language in mind it could be fun and effective, but I'm not sure how much someone would really get out of playing WoW for that reason.