17numbers
54117
1
3
Navigate to System Preferences > Keyboard
Go to the Input Sources tab and hit the "+" to add a new input source
Add the desired input source, in this case, Greek
Optional: a useful shortcut is being able to press-and-hold Caps Lock to switch to a specific (oft used) input source
By default, OSX uses ^Space as a keyboard shortcut for choosing between input sources
When you trigger the keyboard shortcut, you get a quick menu for choosing amongst your input sources