Japanese text displays just fine, but you need to have the correct settings and correct files.
For non-english tags, it is recommended that you use MP4 or M4A files.
Here's a step-by-step of how to enable support.
1) Make sure your language is set to Japanese.
2) In Winamp preferences, you'll need to change your playlist font.
3) Use MP4/M4A files for best results.
and a big thanks to mrym for helping test mp4/unicode issues.