Your account region doesn't matter, just choose a Japanese server. Most JGaffers are on Durandal and Yojimbo.
Remember that you won't be able your NA account with Japanese time cards and that you have to buy the game/expansion (when that comes out) from the same region as your account, so the Japanese retail version codes won't work.
If you're planning to play on a console, you have to start the game from an account from the corresponding region as well, so if your main account is JP, you'll have to switch to a NA one.