CONSTANT needs updating.

\Drupal\language\LanguageNegotiator

updateConfiguration()

LanguageNegotiationUI::METHOD_ID has gone away :-

    foreach ($language_types_info as $type => $info) {
      $configurable = in_array($type, $types);

      // Check whether the language type is unlocked. Only the status of
      // unlocked language types can be toggled between configurable and
      // non-configurable. The default language negotiation settings, if
      // available, are stored in $info['fixed'].
      if (empty($info['locked'])) {
        // If we have a non-locked non-configurable language type without
        // default language negotiation settings, we use the values negotiated
        // for the interface language which should always be available.
        if (!$configurable && !empty($info['fixed'])) {
          $method_weights = array(LanguageNegotiationUI::METHOD_ID);  <-- this line
          $method_weights = array_flip($method_weights);
          $this->saveConfiguration($type, $method_weights);
        }
      }

This issue seems a good starting point for someone at DRUPALCON.

I will be on #drupal-contribute if anyone needs a hand.