Nicolas,
Sorry for the delay - I have been trying to get some of our developers to help look at your request (since I suspect that it may require development to change the current behavior), but it's a very busy period for us.
In the meantime: have you tried adding French translations to the existing taxonomy terms for the levels of description? i.e. navigate to the Levels of description taxonomy, make sure the user interface is flipped to French, enter edit mode, and if there was not already a translation present at installation, then you should see the English terms presented above the edit field as references for your translation.
If that doesn't work (and I suspect it probably won't, unfortunately), then I did poke around the code myself a bit and found this function, that seems to map the EAD level terms to AtoM's English levels of description:
Notice in particular the $variations variable parameters on lines 391-395, as well as the fact that the culture it is explicitly checking against is set to en (English) in line 402.
If you wanted to try modifying the code, this is where I would start. A solution that will work just for your particular needs might involve changing line 402 to fr (French), and replacing the mapped LOD terms in lines 392-395 with your default matching French level terms.
A solution that might work for all and be something to contribute back to the public project would be more complex. I'm not a developer, but I suspect that line 402 would need to be replaced with something that would check the default installation culture of the application as a start.
Anyway, hope this helps. Good luck!