Force Drupal to use English as default language on admin pages.

gistfile1.php

<?php
/**
 * Force Drupal to use English as default language on admin pages and for drush.
 */
if (empty($_REQUEST) || (!empty($_GET['q']) && strpos($_GET['q'], 'admin') !== FALSE)) {
  $language_default = array(
    'language' => 'en', 'name' => 'English', 'native' => 'English',
    'direction' => 0, 'enabled' => 1, 'plurals' => 0, 'formula' => '',
    'domain' => '', 'prefix' => '', 'weight' => 0, 'javascript' => ''
  );
  $conf['language_default'] = (object) $language_default;
}