How do I switch between keyboard layouts?

Its as easy to do as in Lubuntu. Click on System Settings, and click on Keyboard Layout:

Click on the "+" sign to add a layout

Search for the language you want, in this example Greek, and add it

Look at the keyboard layout icon, and you will see the new language. Just click it to choose it.

Ubuntu 12.04

Browse to System Settings -> Keyboard Layout -> + button in the lower left-hand corner of the Keyboard Layout window. Click on the + button and a new window will open that looks like this:

In this example I am using Greek as the new keyboard layout language, but you can substitute any other keyboard layout language in the list for Greek in order to add a different keyboard layout language. Select Greek (polytonic) from the menu and click the Add button to add the Greek Polytonic language to the language selection menu that is in the notification area of the desktop panel next to the volume icon and the clock icon.

Ubuntu 14.04 and later releases

The instructions are the same as for Ubuntu 12.04 (and also for Ubuntu 12.10 and Ubuntu 13.04), except for the first paragraph:

Browse to System Settings -> Text Entr* -> + button in the lower left-hand corner of the Text Entry window. Click on the + button (it's marked by the mouse cursor in the below screenshot) and a new window will open that looks like the above screenshot.

^{Make sure that "Show current input source in the menu bar" is marked by a check mark.}