"Although it can usually work if all you have is the video file, it will be faster (and potentially more accurate) if you have a correctly synchronized "reference" srt file, in which case you can do the following:

subsync reference.srt -i unsynchronized.srt -o synchronized.srt"

I really don't understand what the reference.srt is supposed to be here.

Edit: If I have a "reference" which is synchronized why do I have to do anything at all? Why would I need to synchronize anything then?

Edit2: If the French titles are a translation of the English ones, how wouldn't copying the timestamps from one to another be enough? Or do you want to say that the program can match the subtitles which aren't based on the English ones and have different number of sentences etc? But how can it then know which part of translation belongs to what? What's short in one language isn't in another etc.