I don't want my computers to speak any other language than English, and
I want them to use ISO date formats, so my locale settings have been for
a long time:

$ locale
LANG=
LANGUAGE=
LC_CTYPE=en_US.UTF-8
LC_NUMERIC="POSIX"
LC_TIME=en_DK.UTF-8
LC_COLLATE=POSIX
LC_MONETARY="POSIX"
LC_MESSAGES="POSIX"
LC_PAPER="POSIX"
LC_NAME="POSIX"
LC_ADDRESS="POSIX"
LC_TELEPHONE="POSIX"
LC_MEASUREMENT="POSIX"
LC_IDENTIFICATION="POSIX"
LC_ALL=

This works for most applications :-)
Unfortunately KDE5 does not understand LC_TIME=en_DK.UTF-8 (even if I
select it explicitly in the KDE5 system settings, see attached image.)
Instead it falls back to a
  DD/MM/YYYY HH.MM
short date format whereas that should have been something like
  YYYY-MM-DD HH:MM

Most annoying is the weird '.' time separator in the digital clock
applet (OK, that was fixable with a one line HACK to DigitalClock.qml ...)
since that has a separate switch for ISO date format.
Besides that, I probably don't use enough KDE5 applications that show me
date strings :-)

I'm filing against plasma-desktop since that owns
/usr/lib/x86_64-linux-gnu/qt5/plugins/kcm_formats.so
Please reassign to a more appropriate place if needed.

I haven't tested sid, since I want my desktops to be stable :-)

Andreas