We already know that one of the big goals of Windows Phone in 2011 is expansion: more languages, more markets. In fact, it was stated by Andy Lees that they want 88% of the languages covered this year. Japan is obviously a huge part of that market, so it is not too shocking that Microsoft plans to launch there this year.
In an interview with one of Japan's largest papers, Sankei, Jean-Philippe Courtois, president of Microsoft International said "Japan is next on the list" for support. The big hurdle if implementing the Kana/Kanji writing system, something that takes some time. Adding Japan to the Windows Phone roster will help tremendously to WP7 expansion and adoption.
Meanwhile, there are some 3rd party solutions for those wishing to use Japanese now in an unofficial capacity--see 'Flickpad X2' (homebrew) and 'JPinput' (Marketplace).
Source: Sankei; via Nanapho.jp; Thanks, tezawaly
