For many purposes it is beneficial to use default locale (en_us), since other locales can result in semi-translated UIs (with day-names in your local language whereas everything else is in english). Among others using en_us locale sets the weeks to begin on Sunday. It’s not how we do that in the rest of the world and it's really annoying to see Sunday as a first day of week in a calendar widget across the system. It generally decreases the usability and can even lead to wrong decisions when booking tickets... Currently, only hacky 'solutions' exist (e.g. https://gist.github.com/laineantti/f5e27e1f6a4f13270bbb12c696457542) for something that should be an option. Plasma is supposed to be powerful and configurable.
It seems that there are more users looking to get that feature back. https://www.reddit.com/r/kde/comments/ekb6c3/is_it_possible_to_change_first_day_of_week_from/
Resolved here: https://invent.kde.org/plasma/plasma-workspace/-/merge_requests/405
Not really. That changes the one single view. Not any other app. But if that suffices..