Thanks, JDW.
If I add "LC_CTYPE=ja-JP.UTF-8" into ~/.i18n, then yes, I can choose Japanese input method. But my problem is not yet solved: KMail composer window still ignores keyboard input no matter what language I choose.

Can someone show me your configuration files (/etc/sysconfig/i18n and ~/.i18n) on Mandrake (10.1, 2005, etc.)? It doesn't matter whatever language you use as long as you can type in KMail composer window using SCIM.