hook_node_presave get original of translated node

I'm using a hook_node_presave and I want to detect which field is updated when the translated node is updated. Here is my code :

function hook_import_node_presave(\Drupal\node\NodeInterface $node) {
  if (!$node->isNew()){
    $entityFieldManager = \Drupal::service('entity_field.manager');
    $fields = $entityFieldManager->getFieldDefinitions('node', 'article');
    foreach($fields as $field_name => $field_definition) {
      foreach ($node->getTranslationLanguages() as $langcode => $langcode_value) {
        $node = $node->getTranslation($langcode);
        if ($node->get($field_name)->getValue() != $node->original->get($field_name)->getValue()) {
          // do something
        }
      }
    }
  }
}

However,

$node = $node->getTranslation($langcode);
$node->original->get($field_name)->getValue()

even after "getTranslation()", "original" is still pointing to the default language. I want to get the "original" language of the translated node.