ja: Merge CR v.2 Japanese translation back into main (#2103)

This PR merges the Japanese (ja) v.2 translation back to main.
This requires just a syntactical review, as the partial PRs into this
branch were reviewed already.

You can skim this PR with the [GitHub CLI](https://cli.github.com/):

gh pr diff 2103 | bat -l patch

#1463 #652

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Co-authored-by: Kanta Yamaoka (山岡幹太) <[email protected]>
Co-authored-by: Hidenori Kobayashi <[email protected]>

Author: 3 people