There is English Stack Overflow. There is Spanish Stack Overflow. There is Portuguese Stack Overflow. There is Japanese Stack Overflow. There is Russian Stack Overflow.
But no.. French SO? Why?
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.
L'avis d'un français parmi tant d'autres :p
First, French-speakers only 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 StackOverflow 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 a 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, that's a personal point of view.
The Stack Exchange simply doesn't want to internationalize. I don't know why, and I think maybe also they aren't so sure, as their "explanations".
The German SO proposal was simply deleted by the CM @TimPost on the Area 51, in its commitment phase, after it was held back months long, with a "please wait we are working on it"-like message.
So, they don't want it, and that is all. We are not in the position to change this intent, even if we know it is... well... maybe a little bit sub-optimal.
P.s. On the behavior of the SE I see a pattern, that they seem to want to have only, and exclusively the SO. A single site, on English, and no more. Not the rainbow of the 150 sites of the Stack Exchange, not any non-English content, nothing. Only a single, English-only Stack Overflow. This is what I see, after watching them years long, of course it is only my impression and far from being a sure statement.