Why not set the language for everything to be French?
Unlock the protection, select all, and change proofing language to French, then re-protect the document.
Perhaps that has already been done, but proofing language is set on a character-by-character basis and I just do not know whether the formfields get inserted with a language attribute.

Is there a reason for using legacy fields rather than Content Controls? The protection is far harsher for legacy forms.