In system settings > regional settings > formats, it is possible to set any locale even if it isn't present on the system. This can appear to work while leading to random incomprehensible breakage (eg set LC_TIME to whatever, reboot, see that the KDE clock respects it. However if the locale isn't generated okular will fail to open files with non totally ASCII path like é.pdf, mcomix will just crash on startup, etc)

The program should somehow remind the user to generate locales. Possibilities:
- display only generated locales (maybe with a toggle to display the rest as well)
- display some warning when a non generated locale is chosen
- whatever you come up with