/**
 * Implements hook_language_switch_links_alter().
 */
function {{ machine_name }}_language_switch_links_alter(array &$links, $type, \Drupal\Core\Url $url) {
  $language_interface = \Drupal::languageManager()->getCurrentLanguage();

  if ($type == LanguageInterface::TYPE_CONTENT && isset($links[$language_interface->getId()])) {
    foreach ($links[$language_interface->getId()] as $link) {
      $link['attributes']['class'][] = 'active-language';
    }
  }
}