It should be possible to set this independently of the current locale, just as it was until KDE 4. Please don't go the Gnome way. Reproducible: Always
Thanks for the report We do whatever Qt allows us to do. Qt does not allow us to do a separate setting from the locale. Qt might and hopefully will in the future (5.8?). https://plus.google.com/+JohnLayt/posts/NQCeCRFR3wk
Well, now I have Qt 5.9, and this problem still persists.
Some information is available at https://wiki.qt.io/Locale_Support_in_Qt_5 but currently there is no developer who works on this.