Drupal 8 Address Field – Reorder subfields in an Address field

MODULE_user.module

// Form alter for the User Registration form
function MODULE_user_form_user_register_form_alter(&$form) {
  if (isset($form['field_address'])) {
    $form['field_address']['widget'][0]['address']['#after_build'][] = 'wrf_user_address_after_build';
  }
}

// Custom function for #after_build
function MODULE_user_address_after_build($element, FormStateInterface $form_state) {
  // Move the country field to the bottom
  $element['country_code']['#weight'] = '100';
  return $element;
}