It is on sale on Nuuvem, and a friend of mine has been wanting it for ages.
However, he doesn't speak Portuguese and so I'm wondering whether or not you could either:

a) Change the language settings during the instillation/settings menu
b) Activate the CD Key on GOG and download the English version through here.

Thanks for the help.

A - Yes. Download language packs at the main Witcher site - http://en.thewitcher.com/support/w2/#q29

B - Yes. http://www.gog.com/en/witcher2/backup

Brilliant, thanks!

If you install it via Steam you can change the language as following:

Open Steam, go to your library, then right klick on The Witcher 2
select properties > language
and then change the language in the drop down menu.

If you download the GOG version of The Witcher 2: Assassins of Kings, it actually includes recorded voices in all the different languages. You can select separate voice and subtitle languages from the in-game settings menu. For example, I used Polish voices and English subtitles. I'm not sure if this affects things like the main menu screen text, however; mine was always in English, but I never tried changing the text language to anything else so I don't know whether it works.