The file name is 'the_witcher_3_wild_hunt_polish_language_pack_2_0_0_51.exe' and it is about 1.3 GB in size. You can find it in the Game Goodies section of your library, under The Witcher 3: Wild Hunt - Game of the Year Edition. The first step is to download the Polish language pack from the GOG website. Step 1: Download the Polish Language Pack