perl: warning: Setting locale failed.
perl: warning: Please check that your locale settings:
LANGUAGE = "en_US:de:es",
LC_ALL = (unset),
LANG = "en_DE.UTF-8"
are supported and installed on your system.
perl: warning: Falling back to the standard locale ("C").
de and es were added via plasma config UI
How to set this correctly into the nixos configuration?
and how to test the config before switching (I would mind to get the system state messed up)?
I don’t know whether setting those options will resolve your issue or not, and I’m not sure how to test without switching. That said, rollback is easy with NixOS.