every single french subtitle I've tried doesn't seem to synchronize at all, it's not like it seems to be an issue of timing or something.
Well, if it isn't synchronized, it IS a matter of timing
The problem is, almost all French subs available (
https://www.opensubtitles.org/en/search ... uageid-fre ) are subtitling ONLY the Sicilian and Arabic spoken text, not the French. So, with a lot of text missing PLUS wrong synchronisation, that's why what you describe is happening.
And even if synchronisation would okay for your video file, it wouldn't help you, because you want ALL spoken text (including all French) to be subtitled, right?
Only ONE subtitle is full (
https://www.opensubtitles.org/en/subtit ... prophet-fr ) but to me (with my limited knowledge of French) it seems this is a bad (machine) translation from another language and sync is a bit messy anyway. Have a look at this one. If the French seems okay to you, it is possible to re-sync it to some video version.