Strictly speaking, I think that if you fly into an FR-only airfield without a French language proficiency endorsement on your licence you
could run into trouble if the authorities wanted to make life difficult for you. Whether that would ever actually happen is another matter entirely. It's unlikely, I'd have thought, unless you were involved in an accident. However....
From the text quoted by achimha above:
Epreuves de compétence linguistique
Quand les privilèges de la licence sont exercés dans les espaces aériens où l'usage de la langue française est requis, le titulaire d'une licence doit avoir apporté la preuve d'un niveau 4 ou plus dans cette langue.
If licence privileges are exercised in airspace requiring use of the French language, the licence holder must be able to demonstrate Level 4 or higher in this language
The text continues:
Lorsque cette preuve ne résulte pas du dossier du navigant, le titulaire de la licence doit avoir satisfait à une épreuve complémentaire.
All holders of a French PPL are automatically attributed Level 6 French language proficiency and the endorsement is mentioned on the licence. If you don't hold a French-issued licence I infer from the above that you are required to take a separate French language test
..le titulaire de la licence doit avoir satisfait à une épreuve complémentaire.
Accordingly, the "Compétences linguistiques" section of my DGAC licence mentions
Anglais VFR
Français VFR
Posted at the same time as achimha
