chilly egret
To open the developer menu you have to start the game in dev mode and press ctrl + ~ (which is the left-ctrl-key and the key left from the 1-key below the esc-key).
The game looks at the symbol and not the key itself.
So this does not work on keyboard layouts where the ~ symbol is accessed by using a shift, menu, ctrl, etc... key.
As workaround the user has to change the keyboard layout to us-keyboard every time he wants to open the developer menu and then switch back again to its native keyboard layout.
I attached 2 images showing an ENG-Keyboard layout and an DEU-Keyboard layout.
vague sentinel