Note that these instructions enable the display of Japanese in your Database and Now Playing screens when you have files with Japanese ID3 tags. It will not change the language of your device and your menus will remain in their default language.
Set the Default Codepage to Japanese
Settings > General Settings > Display > Default Codepage > Japanese
Clear the Database to reload all tags from files
Database > Initialize now
Reboot after Initialize finishes in order to incorporate the database changes.
And finally, remember to set a font with Japanese character support:
Settings > Theme Settings > Font > *Sazanami-Mincho
*or any Rockbox fonts that support Japanese.