SUMMARY If "Detailed Settings" is disabled, all the settings under it should be set to the value of "Region". This would be great for quality of life. Right now for me it defaults to en_US and that means that even if in my region Monday is the first day of the week, all plasma calendars will show Sunday as the first day of the week. This can and has probably lead to some confusion (Like what do you mean? My region is correct, yet all the calendars are wrong) and implementing this simple idea would save a lot of clicks and time (At least 10 clicks and a lot of seconds spent trying to find your preferred locale for each setting.) ADDITIONAL INFO OK, I just realized that this has already been suggested in 2015 Bug#344589, but it was closed as NOT_A_BUG after the reportee's valid point was ignored :/ Like yeah? Using region as the value of the greyed out detailed settings is way more sensible and intuitive...
This is effectively fixed in the Plasma 5.24 redesign. I say "effectively" because the UI changed a bit. but I think what you want should be done now.