locale_form_language_admin_overview_form_alter

locale_form_language_admin_overview_form_alter(&$form, FormStateInterface $form_state)

function locale_form_language_admin_overview_form_alter(&$form, FormStateInterface $form_state) {
  $languages = $form['languages']['#languages'];

  $total_strings = \Drupal::service('locale.storage')->countStrings();
  $stats = array_fill_keys(array_keys($languages), array());

  // If we have source strings, count translations and calculate progress.
  if (!empty($total_strings)) {
    $translations = \Drupal::service('locale.storage')->countTranslations();
    foreach ($translations as $langcode => $translated) {
      $stats[$langcode]['translated'] = $translated;
      if ($translated > 0) {
        $stats[$langcode]['ratio'] = round($translated / $total_strings * 100, 2);
      }
    }
  }

  array_splice($form['languages']['#header'], -1, 0, ['translation-interface' => t('Interface translation')]);

  foreach ($languages as $langcode => $language) {
    $stats[$langcode] += array(
      'translated' => 0,
      'ratio' => 0,
    );
    if (!$language->isLocked() && locale_is_translatable($langcode)) {
      $form['languages'][$langcode]['locale_statistics'] = array(
        '#markup' => \Drupal::l(
        t('@translated/@total (@ratio%)', array(
          '@translated' => $stats[$langcode]['translated'],
          '@total' => $total_strings,
          '@ratio' => $stats[$langcode]['ratio'],
        )), 
        new Url('locale.translate_page', array(), array('query' => array('langcode' => $langcode)))
        ),
      );
    }
    else {
      $form['languages'][$langcode]['locale_statistics'] = array(
        '#markup' => t('not applicable'),
      );
    }
    // #type = link doesn't work with #weight on table.
    // reset and set it back after locale_statistics to get it at the right end.
    $operations = $form['languages'][$langcode]['operations'];
    unset($form['languages'][$langcode]['operations']);
    $form['languages'][$langcode]['operations'] = $operations;
  }
}