Per-user language setting

**is your feature request related to a problem? Please describe.**
I'm setting up copyparty on a NAS for my household, and while I would like to keep it in english for myself, to make searching for things easier, some family members absolutely need it to be in our native language (which is luckily supported).

**Describe the idea / solution you'd like**
I would like to be able to set a langauge per user, with the UI getting served in that language.

**Describe any alternatives you've considered**

- #376
but its potentially more difficult to implement, since it hasn't gotten any reply. It might be easier to allow the `--lang` flag to be set to specific users serverside.

**Additional context**