Most pro video editors will let you strip the audio track off once you put the entire movie in the timeline. Once you have removed the original audio track you can add your French version. The first issue will be, how do you know where your new track and movie line up? Do both have sync marks? Are you absolutely certain that both the video and new audio tracks have perfect interval timing? If they don't, what might appear as in sync in the beginning will quickly become totally out of sync in the end.
You can download a trail version of Sony Vegas Pro and use it free for 30 days:
http://www.sonycreativesoftware.com/download/trials/vegaspro
UPDATE:
If you choose Vegas Pro: Put your original movie on the timeline, right click the audio track, in the menu choose "Group" and next to that choose "remove from" then click the audio by itself, then right click and "Delete". This 'group remove from' action allows you to remove the association the video and audio have and thereby allow you to remove the audio.
The next issue will be, did you mean only the voice part? Were you expecting to just substitute the dialogue and keep the music and Foley from the original track? I won't even venture an answer on that part, sounds very tedious and expensive.
As far as the legality. As long as this is for your own viewing, I don't think you will need to worry about the legal part. However, should you show lots of people, or worse publish this on Youtube or something, then yes you can expect that the long arm of the law will reach out and quash it.
Quite honestly, I would just simply hire a very attractive translator and enjoy the movie with her.
