Japanese translation

[koizumistr]

No description provided.

[bact]

The addition of translated files are looking good.

The only warning, which made the check failed, is about the inclusion of those files in nav section of mkdocs.yml.
This needs more discussion later on how we like to deal with it (it is likely that we may need one MkDocs configuration per one language?)
But for now I think this PR is good to merge (and we can improve upon afterward). The translation files themselves are good.

The only question I have in mind now is which branch this PR should merge to?