Journalctl log prefixes lines with month in a non-english language:
monthname 25 15:01:30 ... org.kde.kded.smart[2025]: Detected locale "C" with character encoding "ANSI_X3.4-1968", which is not UTF-8.
Qt depends on a UTF-8 locale, and has switched to "C.UTF-8" instead.
If this causes problems, reconfigure your locale. See the locale(1) manual
for more information.
Running command export LC_TIME=en_US.UTF-8 instantly changes the monthname to Jan (short for January) inside “sudo journalctl -xe” output.
So that is what I wanted. How to set it permanently? I have found file:
sudo nano /etc/locale.conf
which seems to define that exact LC_TIME variable, so it is possibly better to edit inside this file than setting it in a ~/.bashrc file.
So I did that and after reboot, the modification seems to be preserved.
Regarding the “org.kde.kded.smart” warning, it may be caused by mixed locales in command locale;cat /etc/locale.conf output. I do not remember explicitly demanding non-english locale during KDE Neon setup, but I may have selected “non english timezone” and keyboard.