add_filter( 'woocommerce_checkout_fields' , 'override_billing_checkout_fields', 20, 1 );
function override_billing_checkout_fields( $fields ) {

$fields['billing']['billing_first_name']['placeholder'] = 'Prénom';
$fields['billing']['billing_last_name']['placeholder'] = 'Nom';
$fields['billing']['billing_company']['placeholder'] = 'Nom de la société (optionnel)';
$fields['billing']['billing_postcode']['placeholder'] = 'Code postal';
$fields['billing']['billing_phone']['placeholder'] = 'Téléphone';
$fields['billing']['billing_city']['placeholder'] = 'Ville';

$fields['shipping']['shipping_first_name']['placeholder'] = 'Prénom';
$fields['shipping']['shipping_last_name']['placeholder'] = 'Nom';
$fields['shipping']['shipping_company']['placeholder'] = 'Nom de la société (optionnel)';
$fields['shipping']['shipping_postcode']['placeholder'] = 'Code postal';
$fields['shipping']['shipping_phone']['placeholder'] = 'Téléphone';
$fields['shipping']['shipping_city']['placeholder'] = 'Ville';

return $fields;
}

add_filter( 'woocommerce_checkout_fields' , 'override_billing_checkout_fields', 20, 1 );
function override_billing_checkout_fields( $fields ) {
$fields['billing']['billing_phone']['placeholder'] = 'Telefoon';
$fields['billing']['billing_email']['placeholder'] = 'E-mailadres';
return $fields;
}