I have a fresh installation of Debian 11 with XFCE:
locale -a:
C
C.UTF-8
de_DE.utf8
en_US.utf8
POSIX
Only one line modified in /etc/lightdm/lightdm-gtk-greeter.conf:
[greeter]
indicators=~host;~spacer;~clock;~spacer;~session;~language;~a11y;~power
All other lines are commented out.
ligthdm is my current display manager:
systemctl status display-manager:
● lightdm.service - Light Display Manager
However, when I log in, there is no language selection available:
