Japanese World of Warcraft

That's a good question, I'll try to find out.

Knowing that China and Korea both have their dedicated realms and that their languages are extremely different from Japanese I assume that most Japanese players will probably play in English on some oceanic US servers, although the servers themselves are actually located in the US, but this is mostly a wild guess on my part.

Since you're Japanese maybe you know something about this, are you aware of any realms with big communities of Japanese players? Are they in the US, EU, or somewhere else?

Anyway, my guess would be that it's because of Japan’s huge console market, I might be completely wrong though, even though I’m absolutely fascinated by Japan, I’ve never actually been there, so most of the information that I get is from the internet/media and that tells me that the console market surpasses the PC market by miles.
With Diablo being developed for PlayStation maybe things can change, Japan is certainly an interesting market for videogames to say the least.