I think it is not possible. Because the original doesn't have dialog recorded in French maybe due to space constraint at that time. Maybe there mod out there that enable French subtitle but i don't put much hope because this is an old game.

You can use French subtitles (without any kind of mod) in the original broken sword by selecting the language in scummvm, but this will change ONLY the subtitles, if you want the french voice you will have to get the french game since the GOG version only comes with english voices.

Also i do know someone in France with the original broken sword who can supply the speech files so if you can get GOG to give permission for him to upload them i will ask him to do so.