From version 12.1 on, Slackware supports Japanese out of the box

Scim, Anthy, Japanese TTF are ready to go. Check that you are using an utf8 locale (echo $LANG) otherwise set one; GTK2 applications should be scim enabled by default, if not use scim -d. Scim starts default with ctrl+shift.

Because Slackware is Japanese friendly now, the articles below are mostly OBSOLETE.