There are, essentially, two choices here. Which is used would depend on the book, the complexity of the story, and how culturally French these section are.
The translated text could simply all be in French, with understanding that the characters are not actually speaking French for some of the lines of dialog. Descriptive text added around the "incomprehensible" parts explaining the situation. If there are just a few lines, this may be a good option; it'd get very clumsy if there are more than that.
Another way to handle this would be to switch the languages completely. If French is used in the original as simply a random "foreign language", this can work. (A fun example: The 30th-century SF show "Futurama" does this. In the future, French is an incomprehensible dead language. In the French version of the show, the "dead language" is switched to German.) This would be a better option for a longer scene, but it could change the meaning of what's being said if it's tied into it being said in French and not in "random langage that's not what most of the characters speak*.
There's probably no best way to handle this. What's done would depend on the translator's judgment, how much latitude the translator is allowed, and how language-dependent the section is.