This is manly a memo for remember each time which packages are needed for Japanese.

Usually I use xetex with cjk for write latex document on Gentoo this is done by adding xetex and cjk support to texlive.

app-text/texlive cjk xetex app-text/texlive-core cjk xetex

for platex is needed to install

dev-texlive/texlive-langjapanese