The only way to change the display format of date/time, numbers, measurement units, etc., is by changing the locale. That seem very wrong. (Example: how can I switch to 24-hour displays, using ISO formatting for dates, using metric units, etc., without affecting the language settings ?) The localization GUI of the KDE plasma seems to be ill-designed.
That's simply currently how it is designed (and we downstream can't fix that). See also: http://vizzzion.org/blog/2014/05/locale-changes-in-plasma-next/ But some things are possible, example: "Wish: ISO8601 Date Format display option" https://bugs.kde.org/show_bug.cgi?id=348080
Yes, I understand that this is a design bug. But that doesn't make it less of a bug. :-) Thanks for the suggestions, though.