I want to install Greek and English on Raspbian and change the language from the keyboard. Is it possible?
If changing the keyboard layout is all you need, see the question that @goldilocks suggested in his comment. That's what most users want.
If you really need to change the language system-wide (e.g. to accommodate users who don't understand English, or to test software which supports multiple languages), run:
sudo dpkg-reconfigure locales
Select the language you need (along with other locale settings like date format and character set) and restart.
If you want only to change the keyboard layout to Greek:
You can first use the main menu editor to add "LXKeymap" to your application menu. Click on the menu, click preferences, click LXKeymap, and select one of the Greek keyboard layouts. Click apply, and reboot. It should change your keyboard to Greek.
Another way to edit your keyboard layout is to go to the main menu, and under "Preferences", click "Mouse and Keyboard Settings". Then, under the "Keyboard" tab, click "Keyboard Layout" and select your choice of one of the five Greek language layouts.