https://www.interieur.gouv.fr/Actualites/L-actu-du-Ministere/Duree-de-validite-de-la-CNI (mirror) contains a list of countries that are supposed to accept or reject expired French ID cards. It also contains some text explaining the five-year extension translated in the language of each country, which you want to keep a copy of when traveling. Lastly, it advises:
De façon à éviter tout désagrément pendant votre voyage, il vous est fortement recommandé de privilégier l’utilisation d’un passeport valide à une CNI portant une date de fin de validité dépassée, même si elle est considérée par les autorités françaises comme étant toujours en cours de validité.
In order to avoid any inconvenience during your trip, you are strongly recommended to privilege the use of a valid passport to a CNI carrying an expired date of validity, even if it is considered by the French authorities as being always in validity.
FYI: Renewing French ID if living in another country