French native speaker here.

Please see https://xml.insee.fr/schema/commun.html#ChaineFrancaisOfficielMajuscule_stype and https://www.lalanguefrancaise.com/articles/alphabet-francais#liste-des-lettres-de-l-alphabet-en-francais.

The regex you are looking for is :

/^[a-zäàâçéèêëîïôöûùüÿæœ .-]*$/i