Say we want to add the Hellenic (el) keyboard layout in X11. Edit
/etc/X11/xorg.conf
and add the following entries
Option "XkbLayout" "gb,el" # British, Hellenic Option "XkbOptions" "grp:alt_shift_toggle" # Switch with Alt-Shift
Log out / in your X session and try out the configuration by pressing
<Alt>+<Shift>