Ok, I got the polish voice in the game, you have to be quick though:

At the end when the file is downloaded it goes through this "verification" or whatever. While this verification is in progress copy the file pl0.w2speech from your Documents\Witcher 2\Downloads folder into CookedPC folder in your main Wither installation folder.

But you got to be quick, once the verification is over it will delete the file from Documents\Witcher 2\Downloads and start over again.

OR You can download both files from peb.pl (worked for me)

The link is in this thread:
http://www.gog.com/en/forum/the_witcher_2/polish_voiceovers_w_english_subtitles/page3