SUMMARY
There is no en_DE.UTF8 in /etc/locale.gen

STEPS TO REPRODUCE
1. Set main Region to United Kingdom - British
2. Override Currency to Germany - English
3. Apply
4. Logout and Login back
5. test with some program - for example https://archlinux.org/packages/community/any/rebuild-detector/ (command checkrebuild -v)

OBSERVED RESULT
perl: warning: Setting locale failed.
perl: warning: Please check that your locale settings:
        LANGUAGE = "en_GB.utf-8",
        LC_ALL = (unset),
        LC_MONETARY = "en_DE.UTF-8",
        LC_MEASUREMENT = "bg_BG.UTF-8",
        LC_NUMERIC = "bg_BG.UTF-8",
        LANG = "en_GB.UTF-8"
    are supported and installed on your system.
perl: warning: Falling back to a fallback locale ("en_GB.UTF-8").

EXPECTED RESULT
No such error. It messes with possibly important output from other programs as it's printed three times.

SOFTWARE/OS VERSIONS
Operating System: Arch Linux
KDE Plasma Version: 5.23.4
KDE Frameworks Version: 5.88.0
Qt Version: 5.15.2
Kernel Version: 5.15.6-zen2-1-zen (64-bit)
Graphics Platform: Wayland
Processors: 8 × Intel® Core™ i5-8250U CPU @ 1.60GHz
Memory: 7,6 GiB of RAM
Graphics Processor: Mesa Intel® UHD Graphics 620