In the baseFieldDefinitions of Comment Entity, the subject field is set to be translatable, but I can not find such a translation in User interface translation(admin/config/regional/translate) that can make it take effect when comment on a content.

$fields['subject'] = BaseFieldDefinition::create('string')
  ->setSetting('max_length', 64)
  ->setDisplayOptions('form', [
    'type' => 'string_textfield',
    // Default comment body field has weight 20.
    'weight' => 10,
  ->setDisplayConfigurable('form', TRUE);

I now use the code to translate it in my module:

function mymodule_preprocess_form_element(&$variables) {

  if (\Drupal::routeMatch()->getRouteName() == 'entity.node.canonical') {
    if($variables['label']['#title'] == 'Subject') $variables['label']['#title'] = t('Subject');

I don't think it's a good way since the translation is already defined.(And I've defined Hook_Comment_presave, this two function seems will effect each other,which I don’t want it happen either.)

Is there a better way to do that?

  • The label "Subject" (starting with a capital letter) is present in User interface translation out of the box, try this on a fresh install. – 4k4 Jan 26 '19 at 16:50
  • There is "Subject" item in User interface translation, but it it has no effect on a reply comment(may function for a new comment). I use phpstorm/xdebug to trace baseFieldDefinitions of comment Entity, it's not triggered at all (mymodule_preprocess_form_element is triggered). How can I found out which core function really handle this translation then? – xibinke Feb 5 '19 at 16:04

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(&$form, \Drupal\Core\Form\FormStateInterface $form_state, $form_id) {

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



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