Games, in general, are pretty good in making you better at a certain language. For example, when I played the Mass Effect trilogy for the first time, it was hard to understand everything, and in my second time, my english was much better. The social interaction of FFXIV is really interesting and also a poweful tool. When learning another language, even if you study hard, you simply won't get it without trying to practice with other people. I'm now using my english to learn german, so I'm playing FFXIV with german dub and english sub. It has been quite helpful.