Numeric Keypad - switch comma "," to period "." (German keyboard layout)

In Ubuntu 18.04 I couldn't find any appropriate option in "standard" settings (neither in Devices -> Keyboard, nor in Region & Language).

The "clickable" solution I found uses gnome-tweak-tool that I had previously installed for other reasons: sudo apt install gnome-tweak-tool

To run the tool hit Win button and type Tweaks.

In the app in Keyboard & Mouse tab click Additional Layout Options and open the Numeric keypad Delete behavior section. Multiple of the available options there worked for me (I have selected Four-level key with dot as my default).

The nice thing with this solution is that the change is applied instantly as you select the option (no need to click apply or anything). This might be useful if you need to change temporarily to do something and go back to default - just leave the app open do your thing and click back when you are done.

There is an implemented option for kpdl(dot):

setxkbmap de -option kpdl:dot

for permanent changes add the option to /etc/default/keyboard

# KEYBOARD CONFIGURATION FILE

XKBMODEL="pc105"
XKBLAYOUT="de"
XKBVARIANT=""
XKBOPTIONS="kpdl:dot"

BACKSPACE="guess"

This way it will be independent from your xkb-data and safe on dist upgrade.