Use:
use Drupal\Core\Controller\ControllerBase;
use Drupal\Core\Database\Database;
use Drupal\Core\Link;
use Drupal\Core\Url;
use Drupal\Core\Language\Language;
Language Manager:
$languageManager = \Drupal::service('language_manager');
$language_de = $languageManager->getLanguage('de');
URL / Link as HTML: