I would like to set the Caps-lock key as a Compose key, which you do, as far as I remember, by pressing the Options Button in Keyboard Layout > Input Sources and then enabling the appropriate option there. That Button is not pressable though since I switched to 12.10. It did work in previous releases of Ubuntu.

gnome-control-center puts out these errors, when I click on Keyboard Layout:

(gnome-control-center:3645): common-cc-panel-WARNING **: Could not find current language '?\u0003C!\u007f' in the treeview

(gnome-control-center:3645): common-cc-panel-WARNING **: locale '"en_US.UTF-8"' isn't valid

I'm not sure if the errors are related though, maybe they are related to the "interface switching to chinese bug" which seems surprisingly widespread:

Language changed to Chinese, how do I change it back?

Language Support has an unwanted Chinese language option

Nautilus Folders Turned Chinese

Desktop 12.04 gnome/cairo suddenly in Chinese

Unwanted Chinese language got set in system settings

I cannot set my system back to English from Chinese Language

Gnome-classic language turned into Chinese, how do I change it back to English?

Strange display language in gnome shell

I'm not sure they are related to this bug, but I just wanted to mention it, maybe it helps!