"Translate" tab does not appear for every content
The "Translate" tab is handled by the Translation module that comes with Drupal; the Internationalization module that handles the translation for nodes just overrides the page callback for that tab.

function i18n_node_menu_alter(&amp;$items) {
  if (isset($items['node/%node/translate'])) {
    $items['node/%node/translate']['page callback'] = 'i18n_node_translation_overview';
    $items['node/%node/translate']['file'] = 'i18n_node.pages.inc';
    $items['node/%node/translate']['module'] = 'i18n_node';
  }
  // Take over node/add pages for string translation
  $items['node/add']['page callback'] =  'i18n_node_add_page';
  $items['node/add']['file'] = 'i18n_node.pages.inc';
  $items['node/add']['file path'] = drupal_get_path('module', 'i18n_node');
  // @TODO avoid iterating over every router path.
  foreach (node_type_get_types() as $type) {
    $path = 'node/add/' . str_replace('_', '-', $type-&gt;type);
    if (isset($items[$path])) {
      $items[$path]['title callback'] = 'i18n_node_type_name';
      $items[$path]['title arguments'] = array($type-&gt;type, $type-&gt;name);
    }
  }
}


In order to have that tab visible for a node, the multilingual support for the content type must be set to Enabled, with translation.

In Drupal 8, that is done in admin/structure/types/manage/&lt;content type ID&gt;, but the option is titled Enable translation.


  


In Drupal 7, that is done in admin/structure/types/manage/&lt;content type ID&gt;.


  


In Drupal 6, it is done in admin/content/node-type/&lt;content type ID&gt;.