Bug#379052: /etc/default/locale won't work as you expected, seems need more works
Denis suggested dropping /etc/environment and moving locale
information to /etc/default/locale.
I support this idea, but currently this change prevents
the localized desktop environment.
GDM is launched in local language using /etc/default/locale
(called by /etc/init.d/gdm).
But when user logins without any modifications, the desktop
environment will be shown in only English.
This is because LANG isn't defined (GDM_LANG is empty also).
No startup routine care it at this time.
Of course if user chooses language from GDM option and
logins, .dmrc includes LANG information is created,
then the desktop will be shown in choosed language... well,
but I believe such manual doing is not smart.
Although modifying GDM /etc/gdm/Xsession to include
/etc/default/locale file is one of solutions, but I think
it's better that providing a loader script to
/etc/X11/Xsession.d/ and let it run faster.