Did you set your preferred installer option in the dropdown at your NWN account page? It's available in English, French, Italian, German, Spanish and Polish.

Screenshot attached, it's showing as English again on gog.com , but previously it had Polish.. Very odd. Thankfully it's resolved!

You do realise that the white bar saying "English" is a drop down menu where you can select different languages, right?

Hey, I just got the same issue.I downloaded the game from my account (ofcourse it was default English) and after the download finished i tried to install it. The installer then mentioned it is the Polish language...i thought it was weird...
ofcourse I can't read Polish so i had to redownload.While waiting for the second download I searchied the forum and found this post.While casually checking my games and puting a few more for download I checked NWN again and there it was.It had switched to polish version as a default download,without me having done it.I refreshed the page and it switched back to English.
So either there are ghosts fooling around or the gog site has some bugs.