So I am a huge fan of the OST. But, as I play the Polish version (as the gods intended!), the Soundtrack available for download is not quite what I am after. Some of the songs, especially from the expansions, are in English (Main Menu theme, for example). Is there a decent way to download the actual song that plays in my version of the game?

As far as I know, there was no Polish release of the soundtrack. Even the soundtrack included in the Polish boxed version was the English content, with only the CD labelled in Polish. There is also no online provider selling any different content than what is offered on GOG. However, you can still look the songs up on youtube, e.g. the Polish main theme of <i>Blood and Wine</i> and save/convert the stream with proper software on your PC (VLC comes to mind).

Hey, I know this is quite old post, but may help others who are looking for alternative (polish for example) version of songs.

You can extract these songs directly from game files.

Here is how to extract *.wem files from *.cache files and how to convert them to OGG then fix OGG:
https://forums.cdprojektred.com/forum/en/the-witcher-series/the-witcher-3-wild-hunt/mod-discussions/56938-tutorial-sound-extraction-and-basic-modding

I took qbms script from here:
https://mega.nz/#!pFB2kQCJ!4zBa6BDP4cHNwDoaONczdAD-KSb0bo45h-eCZoioqzg

Source post:
https://forums.cdprojektred.com/forum/en/the-witcher-series/the-witcher-3-wild-hunt/mod-discussions/137583-help-extracting-wem-files-from-sounds-cache-specifically-blood-and-wine

For "Blood and Wine" DLC, main menu theme, I found music in The Witcher 3 Wild Hunt\content\patch1\sounds.cache
It's exactly 58588953.wem file.

Unfortunatelly, even if produced OGGs bitrate is 476 kbps, these songs do not sounds so good like FLAC loosless versions from GOG OST download :(

I made quick comparision in Audacity, I imported both tracks, OGG (polish) and FLAC (english) and I was comparing moments without vocal by listening portion of the song from both songs.

I know this is obvious, just saying... I was hoping quality from these OGGs will be better...