This FAQ is as old as Moodle 2.1 as I can see:
https://docs.moodle.org/21/en/Calendar_FAQ
It refers to localewin (for Windows operating systems)
which is a variable in my C:\Bitnami\moodle-3.5.3-0\apps\moodle\moodledata\lang\el\langconfig.php file (not a windows command as I thought).
This file includes the following lines:
$string['locale'] = 'el_GR.UTF-8';
$string['localewin'] = 'Greek_Greece.1253';
$string['localewincharset'] = 'WINDOWS-1253';
$string['oldcharset'] = 'WINDOWS-1253';
which I think are right.
Besides, the FAQ says that BOTH "Names of days and months" are pulled out of your OS.
In my case, names of days are ok! Why/how?
Ralf Hilgenstock that reported similar issue MDL-31622 back in 2012, said that "This is not the Moodle way to code language elements".
Frédéric Massart said "The first patch create lang strings which won't be used very often, probably only for the date selector at that stage, but that's cleaner (IMO)."
David Mudrák said "Without looking into this issue deeply, I would suggest 1) let us try
the reuse the current approach (ie using names provided by locale) as
much as possible. 2) Only when at the end we find that approach too
limiting, we might introduce these names as Moodle native strings and
inject them into YUI."
So, up to now I assume that: 1) It is surely not a priority to fix this, 2) It is a very old and difficult issue, 3) Locale in windows are not always dependable, 4) Lang strings would be a better solution if not to slow things down.