A Japanese wiki is a bad idea, in my opinion. We don't have enough people to do the English wiki alone! (as a note, by the way, wikia does support multilanguage on a single wiki, such as you can see at the Runescape Wiki, but I am against it for our community size, as it would result in poor quality and being largely incomplete).
I think if we want to get more attention we need free advertising. This includes websites like Slashdot, except whatever is popular in your region. It would also probably be nice to have translations available for more languages, including support for different character types, such as Japanese, Simplified Chinese, etc. This would need a few changes to the engine probably though, and specific fonts would have to be used for that one language.