Bug 463165

SUMMARY
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When changing the interface language in Regional Settings from the language chosen at installation to another language, it's impossible to change it back, as the installation language is not present on the list of languages. It's possible to recover the installation language by changing '.config/plasma-localerc', but resulted in odd behaviour (dead keys stopped working, etc.). The only fix found by this user was to delete '.config/plasma-localerc' and restart Plasma. (This was tried on an installation of Kubuntu 22.10, with latest Plasma from backports. The language chosen when installing was Icelandic. I changed the interface language to 'American English' to compare language elements, but Icelandic was not present on the list of languages when I wanted to change it back)

STEPS TO REPRODUCE
1. Open System Settings
2. Select Regional Settings > Region & Language
3. In the entry 'Language', click the 'Modify' button
4. In the 'Language' view, click 'Change Language' and choose a different language from the list
5. Click 'Apply', log out and log in again
6. Repeat the steps above. In step 4., after clicking 'Change Language' the original language should be available on the language list, but it is missing

OBSERVED RESULT
After changing the language from the original installation language to another language, it is not possible to change it back to the original language, as it is not present on the language list

EXPECTED RESULT
Original installation language present on the language list, and should be no problem in changing back and forth.

SOFTWARE/OS VERSIONS
Windows: 
macOS: 
Linux/KDE Plasma: Kubuntu 22.10
(available in About System)
KDE Plasma Version: 5.26.4
KDE Frameworks Version: 5.100.0
Qt Version: 5.15.6

ADDITIONAL INFORMATION
This comes up when the installation language is Icelandic.