How can I recompile the source to force the option to allow changing languages and keyboard layouts at login?
You can explicitly turn on the language selector in the login screen according to this comment:
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.