function wcfm_replace_store_and_shop_texts( $translation, $text, $domain ) {
    $allowed_domains = [
        'bp-better-messages-wcfm'
    ];

    if( in_array( $domain, $allowed_domains ) ) {
        $find = ['Private Message'];
        $replace = ['Contacter le vendeur'];
        $translation = str_ireplace( $find, $replace, $translation );
    }

    return $translation;
}
add_filter('gettext', 'wcfm_replace_store_and_shop_texts', 10, 3);
add_filter('ngettext', 'wcfm_replace_store_and_shop_texts', 10, 3);