Jurato: ... Is this method supposed to work? Am I doing something wrong?...
No, it doesn't work this way, these are the wrong files. triock was only guessing.
You can't get French audio, because these files are missing in GOG's version. Anyway, it's possible to run the game in English with French text and subtitles.
1. Edit FC2Init.ini in the "bin" subfolder, like you did
Set:
language=French
2. Rename these files in the following subfolders:
\Data_Win32\sound_english.dat => sound_french.dat
\Data_Win32\sound_english.fat => sound_french.fat
\Data_Win32\worlds\worlds_english.dat => worlds_french.dat
\Data_Win32\worlds\worlds_english.fat => worlds_french.fat
Done.