Unless "in the process of learning Japanese" means you've taken 2-3 years of college level courses, I don't suggest actually turning on Japanese as your default language. A lot of the quest dialog would be difficult to read, and though I'm sure with a handy dictionary you could get through it, if anyone's waiting on you it's just going to get annoying.
Typing in Japanese is fine though! Thankfully. Something I worried about pre-beta, and am glad it did not turn out like FFXI.