Hi, ja-translators, #652 ! I've refreshed `ja.po` file as of today, Jan
8th 2024.
I used the following procedures. Could you review the diff? Thanks 😊
```
MDBOOK_OUTPUT='{"xgettext": {"pot-file": "messages.pot"}}' \
mdbook build -d po
```
```
msgmerge --update po/ja.po po/messages.pot
```