As from 12.04 the language chooser is in the GTK greeter, which is not included in a fresh Ubuntu installation. In other words you need to install the package lightdm-gtk-greeter separately. Once that's done, you should be able to activate the language chooser in accordance with the above guidance.
Also, in Language Support Help you find a short section about enabling the language chooser on the login screen:
As from 12.04 the language chooser is in the GTK greeter, which is not included in a fresh Ubuntu installation. In other words you need to install the package lightdm-gtk-greeter separately. Once that's done, you should be able to activate the language chooser in accordance with the above guidance.
Also, in Language Support Help you find a short section about enabling the language chooser on the login screen:
yelp ghelp:language- selector# alt-language
HTH