I've recently installed Ubuntu 11.04, which I'm unfamiliar with. I think the language package is installed correctly but there is no layout switching by using alt+shift combination. Tried to look for a solution. Have not found any instructions that work. Please help
I'm not sure if that is what you want. But if you want to change your keyboard layout using Alt+Shift then you could go about it this way:
Go to "Keyboard Preferences" either using the keyboard indicator or dash. Than select the "Layout" pane. There click "Options" which will open the "Keyboard Layout Options". Under "Key(s) to change layout" you can check the box next to your desired shortcut.
I think this is outdated, but maybe worth a tray:
Edit: As stated in the comments: I didn't have a Greek keyboard layout in his layout list and had to add it like this: