Access / Alter user registration form submit button in $form variable
While UserLoginForm is a regular form based on form api, RegisterForm is an entity form, where the actions are defined in a separate method EntityForm::actions(), which is overridden in RegisterForm:

  protected function actions(array $form, FormStateInterface $form_state) {
    $element = parent::actions($form, $form_state);
    $element['submit']['#value'] = $this-&gt;t('Create new account');
    return $element;
  }


which you can override again in your extended class.