Backport "Reset QLocale internals when we have a custom app language"

Original commit:

commit 8361ddef086795cffc43ef9757198642af2439f9
Author: Albert Astals Cid <aacid@kde.org>
Date: Fri Feb 23 01:36:19 2018 +0100

Reset QLocale internals when we have a custom app language

BACKPORT:krita/4.2
(cherry picked from commit a597a8099a3a2ce31bcfe27c6b3dd7f73849e5de)