Why is there no French Stack Overflow?
L'avis d'un français parmi tant d'autres :p
First, French-speakers aren't helpless: there are already many active online French-speaking communities on computing since 1999 (developpez.net for example, in which I am also actively involved).
Moreover, I think that good developers must practice English and I know it's not currently the case, but I've seen that since the increasing use of Stack Overflow or GitHub platforms (among many others recent platforms) around the world, the level of English is growing more and more in the young generation of French developers. And that's really good news. It's even a requirement of our institutions.
Finally, I think the first strength and maybe the main purpose of Stack Overflow is to provide the largest community of IT workers (with a maximum of represented countries), and allow different members of different countries to help each other. And to maintain this purpose, I think that creating a French Stack Overflow would be a really bad idea. Splitting the number of users and potential answers cannot be a good idea.
P.S: I'm not speaking for other French people or other French speakers, this is a personal point of view.
There has been a previous proposal (since deleted) for a French Stack Overflow, and its purpose was discussed at https://area51.meta.stackexchange.com/questions/1280/french-stackoverflow-translation-or-not. However, that link now gives "Page Not Found".
For the current status of Internationalization efforts for Stack Overflow you could review Internationalization 'State of the Stack' - Stack Overflow edition.