This bug is very often posted in forums, but haven't yet found a bug report.

Setting some regional settings in Plasma settings lead to a state where the system locale is defunct. This can cause apps to crash or misbehave. This seem especially common when setting de_DE locales in plasma it seems.

The only way to fix it is to delete the file plasma-localerc, this is even described in the archlinux wiki: https://wiki.archlinux.org/title/Locale#My_system_is_still_using_wrong_language

Here are some user reports:
https://bbs.archlinux.org/viewtopic.php?pid=1435219 
https://www.reddit.com/r/archlinux/comments/udr95x/locale_messed_up_in_kde/
https://www.reddit.com/r/archlinux/comments/uagapq/locale_is_messed_up_im_a_noob_and_unsure_how_to/
https://bbs.archlinux.org/viewtopic.php?id=262047
https://forum.endeavouros.com/t/locale-settings-often-are-messed-up-after-major-plasma-updates/31097
There are a lot more..

So for me an example of a defunct plasma-localerc is:
```
[Formats]
LANG=en_US.UTF-8
LC_ADDRESS=de_DE.UTF-8
LC_NUMERIC=de_DE.UTF-8
LC_TIME=de_DE.UTF-8
```

Then when after a restart when executing the `locale` command:
```
locale: Cannot set LC_ALL to default locale: No such file or directory
LANG=en_US.UTF-8
LC_CTYPE="en_US.UTF-8"
LC_NUMERIC=de_DE.UTF-8
LC_TIME=de_DE.UTF-8
LC_COLLATE="en_US.UTF-8"
LC_MONETARY="en_US.UTF-8"
LC_MESSAGES="en_US.UTF-8"
LC_PAPER="en_US.UTF-8"
LC_NAME="en_US.UTF-8"
LC_ADDRESS="en_US.UTF-8"
LC_TELEPHONE="en_US.UTF-8"
LC_MEASUREMENT="en_US.UTF-8"
LC_IDENTIFICATION="en_US.UTF-8"
LC_ALL=
```

As you see in the first line it says "locale: Cannot set LC_ALL to default locale: No such file or directory".
This error goes away after deleting plasma-localerc, which is the go to method for archlinux users.

This should be fixed in plasma though.