I have the same problem.
I am usingĀ NoMachine 6.0.78_1_amd64.deb on debian Stretch with sddm on the server and client.
As both pcs are in the same room I can access the server with nx as well as with a direct connected keyboard ans monitor.
When I connect with NX the language chooser of the greeter only offers me US for the language (the flag at the top left). That’s specially annoying as I have special chars in my password which are hard to type with a us-keyboard layout.
When I hit any key on the physical attached keyboard the flag (and keyboard layout) changes to German. When I hit a key on the keyboard of the NX client machine it switches back to US. Pretty weird.
After login I can execute “setxkbmap -model pc105 -layout de” and get a German keyboard layout with nx as well as with the locally attached keyboard.