I used to have problem displaying song names in Japanese on Winamp mainly due to Winamp does not support unicode ID tags. Until one day I accidentally stumbled upon this Winamp Plugin Winamp日本語化キット. In English, it simply means Winamp Japanese Locale Kit.

After installation (or during installation), you can choose to have Winamp menus in English with Japanese text support, or simply change the menus to display in Japanese.

This plugin is designed to work for only a specific version of Winamp, so whenever Winamp releases a new version, a new version of the plugin must be installed. Thankfully, the author keeps his plugin up-to-date where a new version of it will be available one or two days after a new Winamp release.