Hi, I want to buy hearts of stone dlc from other site but it says that this is region locked key(ru/cis). I live in Poland and I wanted to know if I could activate hos on my account. I dont use gog galaxy, I wanna download hearts of stone files and thats all because witcher 3 works fine.

1) While GOG does have games you can't buy from certain regions, gift codes for said games should be redeemable from anywhere. Buying a gift code though means paying the normal/high price, not the regional price one, so the ru/cis "buyer" of the gift code would still have to pay the full $9.99, Cheaper it has been is a 15% off, so about $8.49.
2) GOG does not have any sites authorized to sell gift codes. The bonus codes some sites can sell are not regionally locked.

So, if the price for the item is less than $8.49, there is quite a decent chance that you will encounter issues with it down the line. I'd advise against buying it, not because it will be regionally locked, but for other reasons.

I have witcher3 with polish language and if I buy hearts of stone with english language, will I have polish subtitles and dubbing?

I think yes as i can select all the languages.

I bought many keys on g2a and no problems. One time I got the invalid key(without g2a shield) but the seller gave other working key. But its g2a because you can pay with boku mobile.

but why would you buy at a page where you know scammers exist to begin with? By this you willingly accept that the sellers there either way scam the developers directly or some poor lad whose credit card was stolen.
Didn't you get the recent memos?
http://www.pcgamesn.com/action-henk/action-henk-g2a-piracy
http://www.pcgamer.com/tinybuild-claims-g2a-sold-450000-worth-of-its-keys-without-paying-a-penny/

Many people say that, then they return here crying when the scammers' stolen cards get a chargeback and the games get removed in bulk..
Maybe you will be lucky, but don't be surprised if something like this happens.

(and as TT said, it's bad for developers too, because they don't get any money from this)

and another one :P

what really baffles me is that people kinda asking for official support and assistance on their G2A Keys ^^.
I mean ya if you buy at G2A then at least keep silent about it, if you get scammed there especially keep silent about it and learn your lesson - easy as that.
But coming to official developers/sellers homepages and asking for like "how can i get my key from the shady marketplace get to work on which you most likely didn't earn any money and which may even cause you more trouble due to charge-backs dear developer help me pretty please? ... What?

I'm not surprised. Two days ago someone started a thread because he pirated a game and needed help getting it running. When we called him on it he apologized.

Five minutes later he came back and posted a shot of the error message he was getting when he tried to start the pirated game, and once again asked for help. :P

DAMN, I missed such genius : (

There's a region called cis?? It all makes more sense now.........

sounds interesting :D - you have a link to that genius?

https://www.gog.com/forum/general/the_devil_inside