share/debian-edu-config/lightdm-gtk-greeter.conf:
Enable language chooser in the lightdm panel.

etc/X11/Xsession.d/55lightdm_gtk-greeter-rc:
Create environment variables, needed to let the chosen language take effect
everywhere.

share/debian-edu-config/tools/improve-desktop-l10n:
Evaluate configured locales and install related localization packages.