I think playing on a Spanish-speaking server could be a valuable part of your language-learning arsenal. It won't do the trick alone, or even with some phrasebook-ish recordings, but it's far superior for learning Spanish than *not playing on a Spanish server!
If you're interested in going all out, I highly recommend this book for inspiration and technique: http://www.amazon.com/How-Learn-Any-...6577910&sr=8-1 The author outlines his method, which mingles respect for an individual's pace with supremely greedy time-management skills. He goes into good uses for textbooks, foreign media, flashcards, playing WoW on a Spanish-speaking server (OK, no, but it still fits!), etc. ... basically attacking the language on as many different fronts as possible, keeping it interesting and diverse. The book isn't just useful, but I think it'll be a really fun read for anyone interested in learning a second language.
Anyway.I'm a language fan and am happy you're trying something new. Good luck with it!
Connect With Us