Who is required to indicate?

Your answer is correct because both you (the driver) and the blue truck are required to indicate.

According to German traffic rules, when you are turning at an intersection or crossing, you must use your turn signal to communicate your intention to other road users. In this scenario, both you and the blue truck are making turns (you are turning left, and the truck is turning right), so both must use their indicators.

The red car on the left is going straight, so it does not need to signal.

This ensures safety by clearly showing each vehicle's intentions at the intersection, helping prevent misunderstandings or accidents.