On German keyboards is the Point on the Numpad a comma. It ist easier to have a point on the Numpad.
xmodmap -pke > .Xmodmap
sudo nano .Xmodmap
old key is a comma
keycode 91 = KP_Delete KP_Separator KP_Delete KP_Separator
change comma -> point
keycode 91 = KP_Delete period KP_Delete period