A proof that packing under translation of rectangles is NP-complete is given by Richard Korf, Optimal Rectangle Packing: Initial Results. See also this talk by Helmut Alt (search for "PAR is NP-complete"). It follows that polygon packing is NP-hard, but NP-completeness has no been proven in general, as far as I am aware. One special case is proven in Packing 2×2 unit squares into grid polygons is NP-complete