http://www.sphinx-doc.org/ja/stable/intl.html
$ sudo pip install sphinx-intl
conf.py
locale_dirs = ['locale/'] # path is example but recommended. gettext_compact = False # optional.
$ make gettext
$ sphinx-intl update -p _build/locale -l ja
./locale/ja/LC_MESSAGESにpoファイルができるので、これを編集する
$ make -e SPHINXOPTS="-D language='ja'" html