Is there a shortcut for this, or can I create one? I'm running 12.04.
I believe there's no default shortcut. However, you can pick one.
The below screenshots are from Ubuntu 13.04, but 12.04 should be somewhat similar.
Click on the gears icon on the top right and go to System Settings.
Click on Keyboard Layout.
Click on Options....
Expand Key(s) to change layout and pick the key (combination) that you want to use to switch layouts.
If you add more than one layout in "Keyboard Layouts", the top menu bar will show a small icon (indicator) with the current layout, and allow you to change layouts by clicking on it and selecting another one.