Hi,

To contribute to internationalizing the fpm-doc, I forked this repository and translated fpm-doc into Japanese.

Committed changes are as follows:

• created .po files in locale/ja.
• translated all the .po files into Japanese.
• added 'ja' like {%- for lang in ('zh_CN', 'de', 'en', 'es', 'fr', 'ja', 'nl') %} in pages/_templates/sbt-sidebar-footer.html.
• added ja to LANGUAGE in Makefile.
• generated using make html and confirmed pages via some browsers.

Is there anything I should do for internationalization?
Is there any work before sending a pull request?

Thank you.