I spent a little time to analyse the situation and now it’s pretty clear.
All nethserver-lang-<lang> packages contains translations only for the old Server Manager.
So, from the old Server Manager it makes sense having a list of language pack to be installed.
The package nethserver-lang-cockpit (which is built automatically with all other nethserver-lang-*), contains the translations for all supported languages. Such package, is a requirement for nethserver-cockpit, so it’s already installed where the new Server Manager is present.
To recap: language packs are still useful for the old Server Manager, the new Server Manager comes with all languages already installed.
I’m not complaining, just pointing out the list of display languages does not represent the ones nethserver has translations for.
First that comes to mind is Catalan (my language) but I guess it is the same with Suomi and some other languages. For me it’s not a problem as I tend to use it in English (or Spanish if someone else besides me has to use it).