Hey Yuichiro, thanks for this - and cool that you've contributed Japanese translations to Metabase!
We have an open feature request tracking localization support here:
https://github.com/lightdash/lightdash/issues/14109
I've added your suggestion about using a web-based translation platform (Crowdin, POEditor, etc.) to that issue. It's a great point about lowering the barrier so community contributors can jump in without needing Git or PRs. No timeline on this yet. We'll keep you posted as things progress, and we'd love to have contributors like you involved when the time comes 🙂