ja: refresh po file (#2470)

Let me refresh the po file, since I plan to work on some of the fuzzy
translation stuff this week.

This is basically output of only doing: `msgmerge --update po/ja.po
book/xgettext/messages.pot` and removing a few fuzzy tags for obviously
correct translations.

Co-authored-by: Hidenori Kobayashi <[email protected]>