after.php

<?php


class ContentEntityFormController extends EntityFormController {

  public function getFormLangcode(array &$form_state) {
    if (empty($form_state['langcode'])) {
      $form_state['langcode'] = $this->entityManager()->getTranslationFromContext($this->entity)->language()->id;
    }
    return $form_state['langcode'];
  }

  private function entityManager() {
    if (!$this->entityManager) {
      $this->entityManager = \Drupal::entityManager();
    }
    $this->entityManager = $entity_manager;
  }

  /**
   * Optional to simplify unit testing.
   */
  public function setEntityManager(EntityManagerInterface $entity_manager) {
    $this->entityManager = $entity_manager;
  }
}

class BookOutlineForm extends ContentEntityFormController {

  public function __construct(BookManager $book_manager) {
    $this->bookManager = $book_manager;
  }

  public static function create(ContainerInterface $container) {
    return new static(
      $container->get('book.manager')
    );
  }

}

before.php

<?php

class ContentEntityFormController extends EntityFormController {

  public function __construct(EntityManagerInterface $entity_manager) {
    $this->entityManager = $entity_manager;
  }

  public function getFormLangcode(array &$form_state) {
    if (empty($form_state['langcode'])) {
      $form_state['langcode'] = $this->entityManager->getTranslationFromContext($this->entity)->language()->id;
    }
    return $form_state['langcode'];
  }
}

class BookOutlineForm extends ContentEntityFormController {

  public function __construct(EntityManagerInterface $entity_manager, BookManager $book_manager) {
    parent::__construct($entity_manager);
    $this->bookManager = $book_manager;
  }

  public static function create(ContainerInterface $container) {
    return new static(
      $container->get('entity.manager'),
      $container->get('book.manager')
    );
  }

}