>>3 As >>5 said... install the kde-i18n-jp
package.
In Gentoo:
# *echo 'LINGUAS="jp"' >> /etc/make.conf*
# *emerge kde-i18n*
Others, I think have individual packages for the different languages. Or you can just download the source from your local KDE mirror under kde/stable/VERSION/src/kde-i18n/kde-i18n-LANG-VERSION.tar.bz2
.