Add Japanese translations

[stephen-huan]

This is copied directly from Polyglossia rather than Babel because the Polyglossia version is more complete. The only differences are that Polyglossia uses 「端書き」 for Preface (Babel uses 「前書き」) and Polyglossia uses 「文献目録」 for Bibliography while Babel uses 「参考文献」 (which is the same word both use for References). These changes seem reasonable so I defer to the Polyglossia keys for consistency; happy to switch to Babel if necessary.

Is there interest in automatically copying translation data from Polyglossia/Babel with some sort of ad-hoc script? Happy to contribute to something to tools/ if so.

[jgm]

Sure, I think some kind of automatic system could be helpful. There have been some minor (manual) tweaks to some of the translations over the years (could git log data/translations to see).