After a clean install I found out it's by far not fully translated. It seemed to have basic translation copied from the Proxmox GUI.
Currently released for production use is pmg-i18n 1.0-3 which is not fully translated. I just found pmg-i18n 1.0-4 with the (probably) full translation, that was uploaded 5 days ago on no-subscription repository officially not suitable for production use.
This is probably why I unnecessarily made the efforts.