Quick fix for Plasma changing whole language to the same as in regional settings.
What happens: Language setting in Regional settings does not change the interface language, instead the Formats / Region is chosen.
You need to manually modify / add the files in .config: (my language for time/currency is pl_PL, but I default to en_US interface whenever I can):
export LANG=pl_PL export LANGUAGE=en_US
[Formats] LANG=pl_PL.UTF-8 [Translations] LANGUAGE=en_US
Restart Plasma and you should be good to go.