The German AIP, AD 1 - 3 Flight procedures states:
2: Flight Procedures
The traffic circuits for powered airplanes, gliders and ultralight planes (if necessary also for other aeronautical sport products), depicted on the Visual Operation Charts shall be flown. Entry will generally be on the downwind leg, departure on the crosswind leg. If necessary, additional information concerning entries into / departures from the traffic circuit shall be observed.
-----
That's at least what the AIP has to say, most airfields I've flown to are relatively relaxed if you ask to join (e.g.) on base or on a long final, as long as you call them and get clearance to do so and you respect the direction of the circuit. By this I mean, if you are going to land at an airfield operating only a left hand circuit, you can usually ask and expect to get permission to join on base but not right base; however if the airfield has both right and left hand circuits for the same runway, then it is possible to join on base or right base.....