The language packs can be offered as "addon installers", they just add data. I don't know if GOG is going to offer them as full installers or addon packs. One language is about 1.5gigs (mainly the audio).
You can play around with copying the English audio into your French folders, then you'll have English audio and French text. But the French voice overs are on par with the English ones
I.e. once you have installed the French language pack, you could copy
\Data\Localization\English\Soundbanks\*
to
\Data\Localization\French\Soundbanks\*
Now this is all very unofficial and unsupported but it should work