How can I recompile the source to force the option to allow changing languages and keyboard layouts at login?
Did you install the language packs from Language support ?
I seem to have mixed the keyboard layout with the language support!
There is a solution provided in a bug report: Comment #16:
Execute this command:
Find the line:
Change it to:
Save and exit. Logout / Login or reboot the machine to apply the effect.
You can explicitly turn on the language selector in the login screen according to this comment: