Thanks all. I looked at my config files (which go back at least 15 years) and found lots of explicitly setting both LC_ALL=C and LC_LANG=C.
Should I remove all, or just remove the LC_ALL?
> Using LC_ALL is strongly discouraged as it overrides everything. Please use it only when testing and never set it in a startup file.
It is better to set everything explicitly in "/etc/default/locale"
Even better: use
$ sudo dpkg-reconfigure locales
it will generate all locales you need AND set the default one in this file.
There is NO reason to use anything except UTF-8 in 2021