Hello Max,
I don't know KDE/Plasma well, but similarly to other DE, I think it
benefits from being setup from it own tools (KDE System Config or
possibly something similar to dconf for Gnome).
So trying to fix your problem at low level (LC* variables in generic
config files) might not be the most efficient way to solve your
KDE/Plasma problem.
In Trixie 13 KDE/Plasma has been update of a major version (5->6) from
Bookworm 12. Some issues have been documented (menus), potential issues
like yours may appear, I suppose. Hence the suggested use of KDE System
Settings (even if the values are those you expect, try to change them to
something different and then back to the wanted values, to force
KDE/Plasma to acknowledge your setup)
