I'm on windows 8.

I want to buy it, but two of the big reviews on its main page talk of porting issues, while the rest of the forum seem to have its fair share of help requests.

Will I regret buying it because it doesn't work or can I expect a working game?

I think if you don't expect the game to run absolutely flawlessly from scratch, and if you are willing to fiddle a little with compatibility mode and read through forum threads to find solutions to any problems that arise, you are in for quite a treat. The game is a lot of fun and a classic to boot, so you are doing yourself a favor if you buy it. Your mileage may vary, of course.

There's always this : http://www.gog.com/support/website_help/money_back_guarantee

I played DK2 with Win7 at november 2012, only one or two crash on all the sessions.

If the reviews you are seeing are older than that, don't worry.

(I even change the defualt voie and text for the spanish version )

The reviews are pretty old, so don't give much on them. Windows 8 is fully supported by GOG now. Depending on one's system, there can always be little difficulties, it's an old game running on new machines after all. But the game is extensively covered here in the forum, the GOG support is usually very helpful and if nothing helps there's always GOG's money back guarantee.

If you like the game, give it a try. I've enjoyed it and crashes have been seldom.

EDIT: Btw, the many help requests here in the forum, result from DK1 having been a promo-freebie at a point not so long ago. Therefore lots of people grabbed it, thus more forum questions than usual.