Fatal error: Uncaught Error: Class "Drupal\Core\Form\ConfigFormBase" not found in /home/www/drupal8/test/core/modules/language/src/Form/NegotiationSelectedForm.php:14
Stack trace:
#0 {main}
thrown in /home/www/drupal8/test/core/modules/language/src/Form/NegotiationSelectedForm.php on line 14