protected function installParameters() {
  // This will come back with the langcode set to `en`.
  $parameters = parent::installParameters();

  // Update to desired langcode:
  $parameters['parameters']['langcode'] = 'es';

  return $parameters;
}

// Make French the default language.
$edit = [
  'site_default_language' => 'fr',
];
$this->drupalPostForm('admin/config/regional/language', $edit, t('Save configuration'));