Problem
The German umlaut keys, udiaresis: ü, odiuresis: ö and adiaeresis: ä are not recognized as key by the Blender window manager at a GNU/Linux operating system with using Xorg.
Solution
Added German umlaut keys as window manager events and ghost types and mapped the X11 keysymbols accordingly.
System Information
Operating system: GNU/Linux 5.10.34-1-MANJARO x86_64
X11: xorg-server v1.20.11-1
Tested
GNU/Linux X11 with blender-2.82, blender-2.9, blender-292 and latest git master.