Changing language for GDM

[Daniel Díaz <al590349 mail mty itesm mx>]

Hello there!
I recently changed to GDM and I like it a lot.
In the home page you could see some nice screenshots with Czech instead
of English.
How could I do that? I've been looking for a while in the 'net, and found
only Debian-related messages about using /etc/environment, but that doesn't
seem to work.
I'm using Mandrake 9.0, with GDM 2.4.0.11.
I already tried changing the /etc/X11/prefdm script, so that LANG=es_MX
is set before launching gdm, but still with no success.
Any hints?
Thanks in advance!

Daniel Díaz
yosoy danieldiaz org