Since Plasma 5.19.3 (upgraded from 5.19.2) my clock and all other applications changed from the German time format to the US format.

Somehow Plasma does no longer respect my global / system wide settings of LANG/LC_ environment variables despite the fact, that I have selected "no change" and disabled the checkbox "detailed settings" in kcm_formats. 

This is traceable using the commands locale and env inside plasma and on some TTY:

INSIDE PLASMA
$ env | grep -e "LANG\|LC_"
LANGUAGE=
LC_MONETARY=en_US.utf8
LANG=en_US.utf8
LC_MEASUREMENT=en_US.utf8
LC_TIME=en_US.utf8
LC_COLLATE=C
LC_NUMERIC=en_US.utf8

$ locale
LANG=en_US.utf8
LC_CTYPE="en_US.utf8"
LC_NUMERIC=en_US.utf8
LC_TIME=en_US.utf8
LC_COLLATE=C
LC_MONETARY=en_US.utf8
LC_MESSAGES="en_US.utf8"
LC_PAPER="en_US.utf8"
LC_NAME="en_US.utf8"
LC_ADDRESS="en_US.utf8"
LC_TELEPHONE="en_US.utf8"
LC_MEASUREMENT=en_US.utf8
LC_IDENTIFICATION="en_US.utf8"
LC_ALL=


ON TTY
$ env | grep -e "LANG\|LC_"
LANG=en_US.utf8
LC_TIME=de_DE.utf8
LC_COLLATE=C

$ locale
LANG=en_US.utf8
LC_CTYPE="en_US.utf8"
LC_NUMERIC="en_US.utf8"
LC_TIME=de_DE.utf8
LC_COLLATE=C
LC_MONETARY="en_US.utf8"
LC_MESSAGES="en_US.utf8"
LC_PAPER="en_US.utf8"
LC_NAME="en_US.utf8"
LC_ADDRESS="en_US.utf8"
LC_TELEPHONE="en_US.utf8"
LC_MEASUREMENT="en_US.utf8"
LC_IDENTIFICATION="en_US.utf8"
LC_ALL=


I have tried to delete ~/.config/plasma-localerc, but this did not changed anything. The config was recreated using the following settings.

$ cat ~/.config/plasma-localerc 
[Formats]
LANG=en_US.utf8



Operating System: Gentoo Linux
KDE Plasma Version: 5.19.3
KDE Frameworks Version: 5.71.0
Qt Version: 5.15.0
Kernel Version: 5.7.7-gentoo
OS Type: 64-bit
Processors: 8 × Intel® Core™ i7-4790K CPU @ 4.00GHz
Memory: 23.5 GiB of RAM
Graphics Processor: GeForce GTX 970/PCIe/SSE2