Language widget and formatter are now available

A field widget and a field formatter were added for the Language field item, so it is no longer needed to explicitly provide a form element to assign a language to an entity in the related add/edit form. It's also no longer necessary for entity type providers to implement hook_entity_extra_field_info() to describe the Language element.
In fact the related Language entry is automatically made available in the form and view display configuration pages as soon as the Language module is enabled.

Before:

// Entity form
$form['langcode'] = array(
  '#title' => t('Language'),
  '#type' => 'language_select',
  '#default_value' => $node->getUntranslated()->language()->getId(),
  '#languages' => LanguageInterface::STATE_ALL,
  // Language module may expose or hide this element, see language_form_alter().
  '#access' => FALSE,
);

After:

// Entity base field definition
$fields['langcode'] = BaseFieldDefinition::create('language')
  ->setLabel(t('Language'))
  ->setDescription(t('My custom entity language.'))
  ->setRevisionable(TRUE)
  ->setDisplayOptions('view', array(
    'type' => 'hidden',
  ))
  ->setDisplayOptions('form', array(
    'type' => 'language_select',
    'weight' => 2,
  ));