I haven't played an MMO in a long time, but when I played Final Fantasy XI, a large majority of the players were Japanese. The developers included an auto-translate function so you could have something like "Let's go kill [boss name]" and [boss name] would be translated to either English or Japanese depending on the player settings. Once you got familiar with the game and what you did in it, seeing just [stuff] in the brackets got you the gist of what they were saying.

Unless you were trying to learn Japanese though, that's as far as you got -- you couldn't have the seamless "mata a 12 [green dragons]" unless you knew your hiragana/katakana.