Hi roseverte,
Help for translating to Japanese is always welcome, we haven't heard from the company in a while. Please post your weblate.itch.ovh username so I can make you a translation manager for japanese!
Re the homepage: it isn't translated yet, we have plans, but it's not there yet.
Re the Japanese glitch, you can open an issue at:
- https://github.com/itchio/itch.io/issues if it's for the website
- https://github.com/itchio/itch/issues if it's for the app
Thanks!