Change page title (preprocess)

/**
* Implements hook_preprocess_HOOK().
*/
function EXAMPLE_preprocess_page_title(&$variables) {
 $current_path = \Drupal::service('path.current')->getPath();
 if ($current_path == '/user/login') {
   $variables['title'] = 'Login';
 }
 $route_name = \Drupal::routeMatch()->getRouteName();
 if ($route_name == 'commerce_checkout.form' && $variables['title']->getUntranslatedString() == 'Login') {
   $variables['title'] = 'Login/Register';
 }
}