Language chooser in gdm3 not showing new locale

[Curt <curty@free.fr>]

I installed a new locale today because I want to post to a French
newsgroup that does not accept articles in utf8, my default character
set (god knows why).

curty@einstein:~$ locale -a
C
en_US.utf8
fr_FR@euro
fr_FR.iso885915@euro
POSIX

So when I log back in to gnome (I even rebooted like a loser after my
first failure) shouldn't the gdm3 language chooser give me the option of
choosing French as a language now?  Because it doesn't.  It just offers
me English, exactly as it did before I installed the French locale.

What am I missing?  What should I do to make it work?  I did choose
en_US.utf8 as my default locale, but I have the right to do that, don't
I?

Instead of rebooting I should have just restarted gdm3

service gdm3 restart

I don't know what files it sources to get the available locales
(/etc/locale.gen?)

Anyway.