Who is required to indicate?

Your answer is correct: both you (the driver facing the intersection) and the blue truck must indicate.

According to German traffic rules, when turning at an intersection, drivers must always signal their intended direction to inform other road users clearly.

In this situation:
- You are turning left, so you must indicate left.
- The blue truck is turning right, so it must indicate right.
- The red car behind you is going straight and does not need to indicate.

Indicating your turn helps prevent misunderstandings and accidents, especially where multiple vehicles turn in different directions. Thus, both you and the blue truck have to signal your intended turns.