What is the right course of action in this situation?

In this situation, the correct action is to maintain your speed and continue driving as before. According to German traffic rules, when a vehicle in front signals and positions itself to turn left, and there is enough space, you are allowed to carefully overtake them on the right. There is no reason to brake or accelerate unnecessarily. Sudden braking could confuse the traffic behind you, and accelerating is not needed if you are already keeping a safe speed and distance. The most important thing is to stay calm, predictable, and follow the road traffic regulations while overtaking safely on the right. This approach ensures smooth traffic flow and safety for everyone.