Not Quite....
The airline's MEL, which, by law, is at least as restrictive as the manufacturer's MMEL (part of the certification process) is what governs. In the event of an aircraft equipped with two nav lights...a spare, so to speak...and one of the lights is inopt, the aircraft can depart using the second nav light, provided the MEL allows this AND provided an MEL is issued for the inopt nav light.
Yes, that's right....an MEL has to be issued, even though the second (spare) nav light is operational and utilized.
With regard to little airplanes (non-airline aircraft), I have no idea what relief can be granted by FAA. I'd have to read the regs, but, offhand, I'd say you need to get the light fixed before night flight.
I have no idea what JAA allows....
PantLoad