I guess it's a very good place to practice.
There is no secret, to learn a language, you have to practice either by speaking or reading/writting. Considering most of the players speak in english (even when no one in the party is xD) they'll have plenty of occasion to practice ^^
Plus, most of the time, when you don't get what someone is saying, he/she will be nice and rephrase it. Englsih is not my mother tongue and i learned a lot of new things thanks to the game and the players on the forum who would explain to me when they use expression i could not translate ^^