i18n/ja: Add Japanese translation

[akawshi]

Terms that were difficult to translate were translated using Chinese as a reference. Many IT terms were borrowed from English (they are usually written in katakana). In my testing, I noticed that the Japanese display was a bit large, but there was no problem using it.

[bastimeyer]

Thanks for the PR! I appreciate your efforts of adding new translations.

There are a couple of things which need to be fixed. A couple of translation strings were removed on the master branch yesterday, but your PR branch is based on an older commit (218d522), so if I'd merge it in this state, the build would fail. Please rebase onto master, which currently is 2bf7a54 and fix the annotated issues.

I will have a proper review later.

Resolves #961

[bastimeyer]

I had a look at the translations locally and couldn't find any layout issues apart from the settings main menu. I also checked some translations, but not everything. If there's anything unclear, please ask. Otherwise, this should be fine and if something needs further fixing, this can easily be done later on.

That last push unfortunately messed things up and one of the translation removals also removed too much. I will fix this manually on your branch before merging when everything else has been resolved, so that this doesn't make things unnecessarily complicated for you.

Thanks again for your translations.

[bastimeyer]

Thanks again!