> 1. I think "ALT" should be "ALLE" as in "alle filene" (the documentation
> says "ALL" refers to "All files when clock table includes multiple
> files").
>
> 2. I like "Tid stempla", but "Tidsoversyn" or "Tidssamandrag" would stick
> closer to the original and be clearer, I think.
>
> 3. I think "ved" should be omitted. It's not used before dates ("den
> 3.2.2023" but preferably just "3.2.2023"), days ("på fredag") or times
> ("klokka 14:00", "kl. 14:00", "14:00"). It's generally safe to omit it:
> "Tidsoversyn [2023-02-03 Fri 14:00]" would be clear. If a preposition is
> wanted, the entire date could be read "fredag 3.2.2023 kl. 14:00", so
> the "på" before weekday would work, I guess.

That sounds good to me. With those changes we have

  (setq org-clock-clocktable-language-setup
        '(

          ("nn" "Fil"      "N"  "Tidspunkt" "Overskrift" "Tid" "ALLE" "Total tid" "Filtid" "Tidsoversyn")

          ))