I'm using American English in KCM Language and tried de_DE aswell as en_DE in KCM Formats. In the lockscreen the date is always in German language instead of English. For example it's Mittwoch instead of wednesday. The same is true for sddm by the way. But this might be independent of Kscreenlocker. Reproducible: Always
I think that's a Qt problem, @Kai do you know anything?
Yea, probably related to https://bugs.kde.org/show_bug.cgi?id=365139 and the most infamous bug https://bugs.kde.org/show_bug.cgi?id=340982 I just thought it's still useful to create an extra report for the lockscreen aswell. Maybe the reason is here something else or if not, it atleast shows, that the issue is wider spread and not only affecting the digital clock widget. For me personally I find it only to be mildly annoying, but still, it affects several key components of the Plasma desktop.
Replicable on a pair of systems running 5.14.2 even if I'm not using the same combination of locales German/American, got the same symptoms along Bug 365139. The language chose from the original installation stays after the locale is changed system wide from the settings.
*** This bug has been marked as a duplicate of bug 340982 ***