How to change the default date format?

[Marcin Borkowski]

Hello everyone,

I used to use my window manager's language settings, which (presumably)
included LC_ALL and/or other LC_* environment variables, and so I had
the "correct" date format in Emacs when I said
`(format-time-string "%x %X" ...)'.

Now that I've changed my wm to a more bare-bones one, how do I change
the locale/langage settings _from within Emacs_, in my init.el?

By reading through the manual, the docstring of `set-locale-environment'
and some trial and error, I found out about
`(set-locale-environment "pl_PL.utf-8")',
after trying "pl" and "pl_PL" as arguments to no avail.  Is this the
preferred method?

TIA,

--
Marcin Borkowski
http://mbork.pl