Bug 449128

SUMMARY
Plasma does not check whether the configured locale is actually valid/present on the system.

One can set 
```
[Formats]
LANG=broken
```

in `~/.config/plasma-localerc` and Plasma will happily take it without a question.

OBSERVED RESULT
Plasma sets broken locale

EXPECTED RESULT
Plasma warns user that locale is broken through a notification with information how to fix this.

SOFTWARE/OS VERSIONS
Operating System: Arch Linux
KDE Plasma Version: 5.23.5
KDE Frameworks Version: 5.90.0
Qt Version: 5.15.2
Kernel Version: 5.16.2-arch1-1 (64-bit)
Graphics Platform: X11
Processors: 12 × AMD Ryzen 5 3600 6-Core Processor
Memory: 31.3 GiB of RAM
Graphics Processor: NVIDIA GeForce GTX 1080 Ti/PCIe/SSE2

ADDITIONAL INFORMATION

This happens for example when the system has a broken/non-installed language in /etc/locale.conf and the user launches Plasma. Or they later regenerate locales without the language Plasma is configured with.

`localectl list-locales` or `locale -a` could possibly be used for detection of currently installed languages.