Who must signal a change of direction here?

Your answer is correct. According to German traffic rules, the motorcycle must signal its change of direction because it is the vehicle that intends to turn, either left or right, or merge into a new lane.

When you approach an intersection where other vehicles are moving, each driver or rider who wants to change direction must give a clear signal using their indicators. This is to inform other road users and enhance safety by reducing uncertainty about the intended maneuver.

In this situation, since the motorcycle is the one changing direction, it must signal. You, driving straight ahead, do not need to signal because you are continuing on your current path. No one else can signal for the motorcycle, so it is the motorcycle rider's responsibility.

This rule helps prevent accidents by ensuring that all drivers and riders communicate their intentions clearly at intersections and when changing lanes or directions.