We have the translation guideline as [1], and that contains what are
valid Japanese words for the corresponding English words.
This adds a script to verify those words based on the guideline.
[1]: https://kubernetes.io/ja/docs/contribute/localization/
* Add a helper script for localization
The script generates a report of outdated contents in
`content/<l10n-lang>` directory by comparing two l10n
team milestone branches.
* Add description for scripts/diff_l10n_branches.py