FORRAS: https://stackoverflow.com/questions/46326620/custom-placeholder-for-all-woocommerce-checkout-fields

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

     $fields['billing']['billing_first_name']['placeholder'] = 'Keresztnév';
     $fields['billing']['billing_last_name']['placeholder'] = 'Vezetéknév';
     $fields['billing']['billing_company']['placeholder'] = 'Cégnév (nem kötelező)';
     $fields['billing']['billing_postcode']['placeholder'] = 'Irányítószám';
     $fields['billing']['billing_phone']['placeholder'] = 'Telefonszám';
     $fields['billing']['billing_city']['placeholder'] = 'Város';
     $fields['billing']['billing_email']['placeholder'] = 'E-mail cím';


     $fields['shipping']['shipping_first_name']['placeholder'] = 'Keresztnév';
     $fields['shipping']['shipping_last_name']['placeholder'] = 'Vezetéknév';
     $fields['shipping']['shipping_company']['placeholder'] = 'Cégnév (nem kötelező)';
     $fields['shipping']['shipping_postcode']['placeholder'] = 'Irányítószám';
     $fields['shipping']['shipping_phone']['placeholder'] = 'Telefonszám';
     $fields['shipping']['shipping_city']['placeholder'] = 'Város';

     return $fields;
 }