I don’t know what you mean by that. It’s a locale, it has nothing to do with KDE or Plasma. It doesn’t even need a desktop environment. Plasma Settings will just pick up the ones you have installed.

I used to think so too.

However, Plasma apparently has its own list of locales not identical to the system one. (See the first post)

fwiw here’s the en_SE locale source I use http://www.stacken.kth.se/~auno/en_SE as /usr/share/i18n/locales/en_SE so yes, you can add them

This is adding a KDE layout into the system.

Can I add a system layout into KDE?