I’m really not sure what is causing your issue, but a couple of things to try:
- Make sure you have the same
setxkbmapline in the same place on both dom0 and the TemplateVM. - Try hitting Meta + Space - does that switch the keyboard layout?
If those things don’t work, you may have the issue that setxkbmap is not having an effect for keyboards that are plugged in later. So you may have to find a way to run setxkbmap -layout "de" every time the keyboard is plugged in.
The simplest way to do that would be to run that command manually (after the 1st time, you can type Ctrl + r to search for previous commands and start typing setxkbmap). But as you can see in the linked question, there are ways to automate it.
Google or ChatGPT may also have pointers.