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)
List of locales in Plasma ( lemmy.kde.social )
In the Region & Language section of the System Settings, when choosing formats, a list of locales is displayed....