Hello. There is no configuration for keyboard layout, NoMachine gets information about the current keyboard from the client system.
If you are using the most recent version of NoMachine (5.0.63) on client and server it is likely you just need to set the keyboard layout in the remote session. Just run NoMachine, connect to remote session, search for Keyboard configuration (it is located depending on desktop environment in use) and set the French layout. Be sure French is set in the desktop at client side too.
If that isn’t the case, please let me know NoMachine product and version on local and remote machine and whether the problem arises connecting to a physical or a virtual display.