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.

Geia soy Georgios. You don't mention the version of Ubuntu you are using, so I will describe how to add polytonic Greek on Ubuntu 12.04.2 LTS which I'm currently using.

Select the System Settings icon from the dash. Alternatively click dash-home and type 'System settings', then click on the icon presented to you.

On the window that has appeared, in the hardware section, you will notice a keyboard icon. Click on that. The content changes and at the bottom you will notice a link "Layout Settings". Click on that. Again the contents of the window change.

On the bottom left you will notice a "+" sign. Click on that and you will be presented a selection dialog. Type Greek in the search field to narrow down the presented options. Now select "Greek (polytonic)" and click Add. You are done.

If you want you can manage some options and behavior of the new layout by clicking on it and then clicking Options. When done, close the window. You don't need to reboot or re-login for the changes to take affect. Ελπίζω να βοήθησα. ( If this is Greek to you, I just wrote that I hope this was helpful :-) )