Session negotiation settings cannot actually be changed on the UI

<?php
  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state) {
    $config = $this->config('language.negotiation');
    $form['language_negotiation_session_param'] = array(
      '#title'=> $this->t('Request/session parameter'),
      '#type'=> 'textfield',
      '#default_value'=> $config->get('session.parameter'),
      '#description'=> $this->t('Name of the request/session parameter used to determine the desired language.'),
    );
    $form_state->setRedirect('language.negotiation');
    return parent::buildForm($form, $form_state);
  }
  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {
    $this->config('language.settings')
      ->set('session.parameter', $form_state->getValue('language_negotiation_session_param'))
      ->save();
    parent::submitForm($form, $form_state);
  }
?>