Changing the keyboard layout turns off the numlock

Turns out this is a known bug and the bug report includes a patch. You could try applying the patch and recompiling X or you could try this workaround:

1. Save this script as ~/bin/toggle_keyboard.sh

   #!/bin/bash 
   
   current=`setxkbmap -query | grep layout | fold -s2 | tail -n 1`
   if [ "$current" == 'us' ]
   then
       setxkbmap -layout gr
   else
       setxkbmap -layout us
   fi
   

2. Make the script executable with chmod +x ~/bin/toggle_keyboard.sh.

3. Go to Settings => Keyboard => Shortcuts => Custom Shortcuts and assign create a shortcut that runs /home/stelios/bin/toggle_keyboard.sh (obviously, change stelios to whatever your actual username is).

You should now be able to use this new shortcut to toggle between your keyboard layouts.