In the Region & Language section of the System Settings, when choosing formats, a list of locales is displayed.

The entries of this list are similar to those in the system (e.g. /usr/share/i18n/supported in Debian) but not identical.

E.g. en_DK.UTF-8 is present in the system but absent from the Plasma locale list; en_SE.UTF-8 is present in the Plasma list, but absent from the system.

Why does Plasma use a separate list of locales?

Where is the Plasma locale list located in the file system?

Is it possible to add a locale absent in Plasma from the system to Plasma?