I remember that I read somewhere that the US version is missing DRM. Not sure about that though. I will do some research right now ;)
EDIT: "It appears the North American version of this game does not have any copy protection, and hence has no DVD check either. Hopefully THQ will remove the CD/DVD check component of the copy protection for other regions in a future patch."
- http://www.tweakguides.com/STALKER_4.html
So this could mean the downgrade might work with the US-Patch and correct Lang Code

I emailed GOG support to see if they could help with downgrade instructions. The couldn't but the request has been passed along to the product team and they will be investigating the possibility of a proper GOG downgrade patch. Just have to be patient.
There are a few threads/questions about this in this forum. They all have pretty much the same info. So id imagine that GOG are aware of the problem.

@TheMechanist
That works perfectly! You sir, are my hero!

gamershell.com/download_21947.shtml
US V1.0005 Patch

In reg edit Change "install source" to:
stk-for-pack-noprot-efis
and
"InstallLang" to:
en

and your good to go.

also works for the US v1.0004 patch.