Plasma uses $HOME/.config/plasma-localerc, you might want to adjust that one:
$ cat ~/.config/plasma-localerc
[Formats]
LANG=de_DE.UTF-8
[Translations]
LANGUAGE=de:en_US
There also is $HOME/.config/plasma-locale-settings.sh, but it has comments saying it’s generated:
$ cat ~/.config/plasma-locale-settings.sh
# Generated script, do not edit
# Exports language-format specific env vars from startkde
# This script has been generated from kcmshell5 formats.
# It will automatically be overwritten from there.
export LANG=de_DE.UTF-8