Here is one way to make subtitles for your movie:
There are other tools for working with subtitles, like Subtitle Workshop and Jubler.
This is not an easy process, and you'll need to be able to translate the French sections. Sometimes you can find the entire script of the movie online at sites like http://www.simplyscripts.com/
Once you have the .srt file, you'll need to reauthor the dvd and add the subtitle stream. You can use a lot of different tools for this, including DVDStyler or GUIforDVDAuthor.