How to set the tabs on node pages to always display in the site's default language?
You can use this function (inspired from this solution wich was for Drupal 6)

/**
 * Reverse translation: find source string for a given translated string.
 */
function rt($string) {
  global $language;
  $langcode = $language-&gt;language;
  $query = db_select('locales_source', 'ls');
  $query-&gt;leftJoin('locales_target', 'lt', 'ls.lid = lt.lid');
  $query-&gt;fields('ls', array('source'));
  $query-&gt;condition('lt.language', $langcode);
  $query-&gt;condition('lt.translation', $string);
  $result = $query-&gt;execute();
  $records = $result-&gt;fetchAssoc();

  return $records ? $records['source'] : $string;
}


Then via hook_menu_local_tasks_alter

/**
 * Implements hook_menu_local_tasks_alter().
 */
function mymodule_menu_local_tasks_alter(&amp;$data, $router_item, $root_path) {
  foreach ($router_item['page_arguments'] as $key =&gt; $argument) {
    if (is_object($argument) &amp;&amp; isset($argument-&gt;type)) {
      foreach ($data['tabs'][0]['output'] as $key =&gt; $value) {
        $data['tabs'][0]['output'][$key]['#link']['title'] = rt($value['#link']['title']);
      }
    }
  }
}