Live captions in French do not work for Facetime

I bought an iPad Air 5 for my deaf grandma so I can call her on Facetime and she can read what I say.

I set the language of the device to French, and the region to France. I also selected the setting to always display live captions in Facetime.

When starting a call, I briefly got a modal telling me a language model for French was about to be downloaded. I think it failed (way too fast, the english one took more time), but I never saw this modal anymore.

On a call, live captions does not work. If I set the region to USA (but keep the language of the device to French), live caption works, but... in English only.

How can I get this to work in French?