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):

~/.config/plasma-locale-settings.sh

export LANG=pl_PL
export LANGUAGE=en_US

~/.config/plasma-localerc

[Formats]
LANG=pl_PL.UTF-8
[Translations]
LANGUAGE=en_US

Restart Plasma and you should be good to go.