In this situation, does the speed limit apply only to the outermost right lane?

Why?

The B-33 sign showing 50 stands on the right-hand side of the carriageway, so the limit applies to all lanes travelling in your direction. The separate blue sign below it shows the lane layout and does not restrict the speed limit to the right-hand lane. A sign applying only to a particular lane would be positioned above that lane.

§ 2 ust. 1 rozporządzenia w sprawie znaków i sygnałów drogowych; B-33 „ograniczenie prędkości”