Hi
Welcome to the NethServer forum!
As said, this is the NethServer Forum. I don’t quite see what Proxmox Backup Server together with NTFS language support (For Windows) has to do with NethServer?
A lot of people here do use Proxmox and also PBS, myself included, but if you have any requests about PBS, it would be better to go to the PBS forum…
PBS itself does support russian (and cyrillic), as I can see on my home node…
Maybe not yet 100%, but that is more likely due to a lack of contributions by cyrillic language speakers… There are a lot of languages, but I also do not see serbo-croatic, another also cyrillic language…
It’s open source! Don’t bitch about it, but contribute if you can! And if you need cyrillic, you can! 
My 2 cents
Andy