I downloaded GrblGru Ver.3.45.10,
Language_jp.txt and QuickHelp_jp.txt from Language_jp.zip
By copying to \\ProgramData\GrblGru\Language directory, GrblGru Ver.3.45.10 can be changed to Japanese menu.

I do not know the author of the Japanese file.
However, it seems that you can log in to this site from "Login" at the following URL.
If you translate the page with Google Translate, you will see the letter "Login" third from the bottom left.https://pc-trace.jpn.org/blog/blog.cgi?n=871