Most notable is keyboard layout. In different VMs current layout may differs, and there is no correct indication of selected layout.
(I mean in the area bottom right, near to the clock)
Indeed. But dom0 have no knowledge about internal VM keyboard layout. You can
request (from qubes-manager) keyboard layout for VM, but VM can ignore it or
set something different (eg. when you use some keyboard changing program in VM).
The other problem is the standard KDE controls have no way to change keyboard
layout inside of VM. So this icon basically means "default keyboard layout".
Anyway it's good idea to at least indicate dom0-requested keyboard layout.
Perhaps this is as simple as some window property (xprop tool can be useful
here). Patches welcomed.