In this situation, having completed overtaking, are you obliged to return to the right traffic lane?

Yes, after completing an overtaking manoeuvre, the driver is obliged to return to the right-hand lane immediately. According to Art. 16 para. 4 of the Road Traffic Act, in Poland, you must drive on the right, and the left lane on a road with at least two lanes in one direction is primarily for overtaking. Persistently driving in the left lane when the right one is free is an offence because it obstructs traffic for other vehicles.