There is option in system settings to change the ordering of days of the week in the calendar widgets. Locale alone isn't sufficient if you for example want to use date and time format from one locale and days of the week ordering from another locale. Please add a separate setting.
I meant there is *no option currently.
Locale data comes from the CLDR database inside Qt, and is currently not customizable.
The(In reply to Christoph Feck from comment #2) > Locale data comes from the CLDR database inside Qt, and is currently not > customizable. Then I suppose this should be seen as a deficiency that can to be somehow addressed either in Qt or in KDE itself to allow bypassing Qt limitations. If you think it makes sense to open a Qt bug about it upstream, I can file one.
Qt developers are aware of the defiencies, but lack a contributor to rework the locale code. For more information, please see https://wiki.qt.io/Locale_Support_in_Qt_5 Bypassing means that we would re-add our own KLocale classes (what we had for KDELibs4 applications), which we want to avoid. Users were complaining that KDE applications weren't following the system's locale, but had their own configuration. POSIX could adopt CDLR; Qt then wouldn't need it's own database. At least on Linux.
So yes, it needs to be fixed in Qt. Nothing we can do here, sorry. :)