Genko, can you read/write hiragana/katakana/kanji? What kind of Japanese are you trying to learn? What's your purpose in learning Japanese?
Depending on what you can do / want to do, learning Japanese in FFXI may not be a very good option. The Japanese used by many of the players is equivalent to the English used by many people you'll meet in the game - it's not going to be the same as the language used in business practice or everyday conversation, for example. (Admittedly, much like English players you'll find some who speak normally and some who "sp34k liek nubs", so you could get lucky...)
I'd as in Whitegate...but I can't really form a JP sentence
and I have idea what to word with auto-trans.
Considering what you said above, I'd probably strongly advise learning to read/write coherent sentences first, and THEN try to implement FFXI as a practice environment, if you still decide to do so.