Why the label of "subject' field in comment-form not be translated and how can I do the translation in a better way?
I finally found a solution to enforce translation of 'Subject' label in hook_form_alter which can make code more clear:

function mymodule_form_alter(&amp;$form, \Drupal\Core\Form\FormStateInterface $form_state, $form_id) {

  if($form_id == 'comment_comment_forum_form') {
    $form['subject']['widget'][0]['value']['#title'] = t('Subject');

  }

}