Difference between nominal speed and idle RPM of a DC generator
Idle speed usually means the speed at which the generator should be driven by a prime mover when there is no electrical load. A generator with no electrical load is easier to turn mechanically, so unlike your car, its idle speed is faster than its nominal speed.

An unladen generator is faster, but not as useful because it does nothing. You will want to run it at nominal (laden) speed and torque (regardless of whether it is a European or African generator), which is the speed-torque combination that provides peak efficiency for a given electrical load. The speed of the prime mover and the impedance of the electrical load are the variables you can adjust to hit maximum efficiency. Often, one cannot control the impedance, so the speed of the prime mover (perhaps a petrol engine) may be the only adjustable variable.

The nominal speed of a DC generator does not necessary match the national power supply frequency, because a DC generator outputs DC. If you use a DC generator, it's to create a DC supply for some reason, such as generating a high voltage supply to create an ex-parrot (the technical term being "voom").