Drupal 7, search in current language.

my_search_module.php

<?php 
/**
 * Implements of hook_query_node_access_alter().
 */
function my_search_module_query_node_access_alter(QueryAlterableInterface $query) {
  $db_and = db_and();
  $language = i18n_language_interface();

  $db_and->condition($node . '.language', $language->language, '=');
  $query->condition($db_and);
}