In the presented situation, is it mandatory to stop before entering this intersection?

In the video you are approaching a roundabout whose island is decorated with flowers; on the right there are signs A-7 "give way" and C-12 "roundabout", on the traffic island there is C-9, and in front of you a red car is entering the roundabout. When entering this junction, you do not have to make a mandatory stop - you only have to give way. The answer is: no.

The Regulation on road signs and signals states that sign A-7 "give way" warns of a junction with a priority road, while a mandatory stop is required by a different sign - B-20 "STOP"; sign C-12 with A-7 means that a driver already on the roundabout has right of way over one who is entering. Giving way means refraining from moving only when your movement could force another driver to change direction or lane or to change speed significantly. If the roundabout is empty, you enter without stopping, only slowing down; if someone is circulating on the roundabout from the left or, as here, the car in front of you brakes before the entry, you slow down and, if necessary, stop before the give-way line. A mandatory stop at an empty roundabout holds up traffic and surprises the drivers behind you.

Legal basis: § 5 para. 5, § 21 para. 1 and § 36 para. 2 of the Regulation on road signs and signals; Art. 2 point 23 of the Road Traffic Act (Prawo o ruchu drogowym)