Hey, there is Norwegian available for selection in the login screen. Unfortunately the language doesn't seem to be really maintained currently. If you want to help to change that, get familiar with the way the development is handled here: https://pve.proxmox.com/wiki/Developer_Documentation
I started reading, but this looked like a lot more work than just the translation-part - since I'm not at all familiar with git If you could send me (and I'll update the file) or I could do it in my install, that would have been the only way I would have had time I think.
The git repository web interface gives you a browse option through which you can download individual files. They are stored in PO format, which is extremely useful for translation work because it offers nice features to deal with outdated translations, and if you want to work on the translations you should get yourself familiar with it.
Though they are getting pre-processed for being used by the web interface, and that used format is a bit more complex and not really useful for direct editing. You can find out a bit more on this in the wiki: https://pve.proxmox.com/wiki/Translations - for the mail gateway you have to replace "pve" in the snippets at the top with "pmg". We are welcoming translation contributions, given that most people working here only speak English and German, with a few more.