I will ask, but there aren't many elders left from that period.

I put on my speleologist's outfit and continued my research :
I found this topic where Davout was the first moderator of the french section on June 21 2011 (the message from elelegzet confirms it and this one too). And on June 22 Davout asked for the creation of new sub-sections.

And I found this topic where on February 18 2011 grondilu says:
"For such a technical question you should really post in english on the support forum."
So we can assume (but I have no certitude) that the french section already existed on that date.

For fun, there is thistopic where Gavin Andresen met French people in Paris (too bad the links of the pictures are dead). But if he apologizes for not being able to write in french, it would confirm the existence of the french section on that date.

Sad that Webarchive doesn't go back before October 13 2011: https://web.archive.org/web/20111013175345/https://bitcointalk.org/index.php?board=13.0

But for me the french section was created at the end of January / beginning of February 2011.

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Edit :
The creation of the French section may not have happened in January 2011. Saint-Loup has found an interesting message :