Checking if module has the right output on multilingual pages with simpletest_clone
I would set the default langauge in code first instead.

// Get the list of languages
$languages = language_list();

// Set up the new language code
$new_lang_code = 'fr';

// Make sure the required language object is actually set
if (isset($languages[$new_lang_code])) {
  // Overwrite the global language object
  global $language;
  $language = $languages[$new_lang_code];
}


and then you just need to get the front page (and Drupal should add the prefix).

$this-&gt;drupalGet('&lt;front&gt;');


If front doesn't work you might need

$front_url = variable_get('site_frontpage', 'node');
$this-&gt;drupalGet($front_url);