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blabla3: I have steam version of this game but I would like to buy GOG version...I can change language with steam easily but my friend purchased GOG version yesterday and he said that game is only in english, no matter he picked up French during installation(!)

Can I change language with GOG version and how?

P.S. To my friend it seems that game picks up language from Windows...this is no good because some people have OS in another language than language they wish to use in games! :(
If he downloaded offline installers via browser, files should have '_french' in their name. For offline setups, each language has a separate installer.

If he has GOG Galaxy, language can be switched in Galaxy settings by going to game page and picking MORE -> SETTINGS section.

To my knowledge, OS language makes no difference in this case. It's possible that he installed the game in French but then Galaxy sync'ed it to English due to some glitch, but then it should be easily switchable back in the Client
Does Steam version of DDDA allows separate text / voice selection?
kingit: Yes. Not ingame but you can select your game in the library, properties and then change language in language tab. You do not have to download anything. Game voices are english only anyway, only interface and subtitles are changed with this. Plenty of other games language is changed this way...
Same can be done via Galaxy Client, for DDDA at least.
Separate installers are available as a convenience for users who prefer not to use the Client at all.