Turns out that all you have to do is switch the "sounds" folder from the language version you wish to play in, with the "sounds" folder in the english version of the game (you can install different versions by clicking options, then preferred language in the gog installer)

So right now I'm playing the english game with french speaking characters, and it works just fine!
I think...
I've just begun playing and havn't actually gotten anywhere yet and the only thing I'm worried about this "fix" is if cutscenes will have french audio ( I assume) but if they also will be subtitled.
Some other quick gamer might confirm if this works?

Go the other way round - Install the French version and replace the following files from the files of the English version:
data / string.dat
data/font_syberia2.dat
textures / InGame

For those files in the textures / InGame are not the used all files, but I think it will be easier that way.

All audios should be French (even cutscenes) and subtitles shoul by English.

I wanted to know if I could play Syberia in French with Italian subtitles. The solutions that you two suggested are very clever. I will try to do as standa.e suggests. I think that being a French game set in a French town, although inhabited by automs, playing it in the original language adds to the atmosphere. But my French is far to be adequate o the task without the help of subs. I hope I will also learn something. I have played the first Syberia on disc many years ago, in Italian, and I can now play all the episodes here on GOG. :)
Thank you guys!