class LanguageFormBase

abstract class LanguageFormBase extends EntityForm {
    
    /**
     * The configurable language manager.
     *
     * @var \Drupal\language\ConfigurableLanguageManagerInterface
     */
    protected $languageManager;
    
    /**
     * Constructs a ContentEntityForm object.
     *
     * @param \Drupal\language\ConfigurableLanguageManagerInterface $language_manager
     *   The configurable language manager.
     */
    public function __construct(ConfigurableLanguageManagerInterface $language_manager) {
        $this->languageManager = $language_manager;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function create(ContainerInterface $container) {
        return new static($container->get('language_manager'));
    }
    
    /**
     * Common elements of the language addition and editing form.
     */
    public function commonForm(array &$form) {
        
        /** @var \Drupal\language\ConfigurableLanguageInterface $language */
        $language = $this->entity;
        if ($language->getId()) {
            $form['langcode_view'] = [
                '#type' => 'item',
                '#title' => $this->t('Language code'),
                '#markup' => $language->id(),
            ];
            $form['langcode'] = [
                '#type' => 'value',
                '#value' => $language->id(),
            ];
        }
        else {
            $form['langcode'] = [
                '#type' => 'textfield',
                '#title' => $this->t('Language code'),
                '#maxlength' => 12,
                '#required' => TRUE,
                '#default_value' => '',
                '#disabled' => FALSE,
                '#description' => $this->t('Use language codes as <a href=":w3ctags">defined by the W3C</a> for interoperability. <em>Examples: "en", "en-gb" and "zh-hant".</em>', [
                    ':w3ctags' => 'http://www.w3.org/International/articles/language-tags/',
                ]),
            ];
        }
        $form['label'] = [
            '#type' => 'textfield',
            '#title' => $this->t('Language name'),
            '#maxlength' => 64,
            '#default_value' => $language->label(),
            '#required' => TRUE,
        ];
        $form['direction'] = [
            '#type' => 'radios',
            '#title' => $this->t('Direction'),
            '#required' => TRUE,
            '#description' => $this->t('Direction that text in this language is presented.'),
            '#default_value' => $language->getDirection(),
            '#options' => [
                LanguageInterface::DIRECTION_LTR => $this->t('Left to right'),
                LanguageInterface::DIRECTION_RTL => $this->t('Right to left'),
            ],
        ];
        return $form;
    }
    
    /**
     * Validates the language editing element.
     */
    public function validateCommon(array $form, FormStateInterface $form_state) {
        // Ensure sane field values for langcode and name.
        if (!isset($form['langcode_view']) && !preg_match('@^' . LanguageInterface::VALID_LANGCODE_REGEX . '$@', $form_state->getValue('langcode'))) {
            $form_state->setErrorByName('langcode', $this->t('%field must be a valid language tag as <a href=":url">defined by the W3C</a>.', [
                '%field' => $form['langcode']['#title'],
                ':url' => 'http://www.w3.org/International/articles/language-tags/',
            ]));
        }
        if ($form_state->getValue('label') != Html::escape($form_state->getValue('label'))) {
            $form_state->setErrorByName('label', $this->t('%field cannot contain any markup.', [
                '%field' => $form['label']['#title'],
            ]));
        }
    }

}