To write Japanese text with AUC TeX you need to have versions of TeX and Emacs that supports Japanese. There exist at least two variants of TeX for Japanese text, and AUC TeX can be used with both, as well as with the two Japanese-aware Emacses, NEMACS and MULE.
To use the Japanese TeX variants, simply enter japanese-tex-mode,
japanese-latex-mode, or japanese-slitex-mode, and
everything should work. If not, send mail to Shinji Kobayashi
(`<koba@keisu-s.t.u-tokyo.ac.jp>', who kindly donated the code for
supporting Japanese in AUC TeX. None of the primary AUC TeX
maintainers understand Japanese, so they can not help you.