Custom Widget Setting

I am currently porting the amazon_pa module to drupal 8: https://www.drupal.org/project/amazon_pa

The widget has a custom setting called locale. Meaning it is a select box here: /admin/structure/types/manage/article/fields/node.article.field_amz/storage

I am not able to save the vale i input there. So what one has to do that the form element $element['locale'] is actually saved and can be retrived late?

Thank you very much.

<?php

namespace Drupal\asin\Plugin\Field\FieldType;

use Drupal\Core\Field\FieldItemBase;
use Drupal\Core\Field\FieldItemInterface;
use Drupal\Core\Field\FieldStorageDefinitionInterface;
use Drupal\Core\TypedData\DataDefinition;
use Drupal\Core\Form\FormStateInterface;




/**
 * Plugin implementation of the 'asin' field type.
 *
 * @FieldType(
 *   id = "field_asin",
 *   label = @Translation("Amazon asin field"),
 *   module = "asin",
 *   description = @Translation("Amazon ASIN field"),
 *   default_widget = "asin_text",
 *   default_formatter = "asin_plain"
 * )
 */

class AmazonField extends FieldItemBase {

  /**
   * {@inheritdoc}
   */
  public static function schema(FieldStorageDefinitionInterface $field_definition) {

    $locale = $field_definition->getSetting('locale');

    return array(
      'columns' => array(
        'asin' => array(
          'type' => 'varchar',
          'length' => 32,
          'not null' => TRUE,
        )
      )
    );
  }

  /**
   * {@inheritdoc}
   */
  public function isEmpty() {
    $value = $this->get('asin')->getValue();
    return $value === NULL || $value === '';
  }

  /**
   * {@inheritdoc}
   */
  public static function propertyDefinitions(FieldStorageDefinitionInterface $field_definition) {
    $properties['asin'] = DataDefinition::create('string')
      ->setLabel(t('ASIN value'));

    return $properties;
  }


  /**
   * {@inheritdoc}
   *
   * Add locale settings to a field
   */
  public function storageSettingsForm(array &$form, FormStateInterface $form_state, $has_data) {
    $element = [];
    $settings = $this->getSettings();

    $Utils = New \Drupal\amazon_pa\Utils\AmazonPaUtils();
    $cache= $Utils->amazon_pa_data_cache(FALSE);
    $config = \Drupal::config('amazon_pa.settings');

    $locale_options = array('' => '-- Select --');
    foreach ($cache['locales'] as $locale => $data) {
      if ($config->get('amazon_locale_' . $locale . '_associate_id', '')) {
        $locale_options[$locale] = $data['name'];
      }
    }

    $stop=1;

    $locale = $this->getSetting('locale');

    $element['locale'] = array(
      '#title' => t('Amazon Locale'),
      '#type' => 'select',
      '#options' => $locale_options,
      '#default_value' => $this->getSetting('locale'),
      '#description' => t('Madatory locale'),
      '#disabled' => $has_data,
      '#required' => TRUE,  

  
    return $element + parent::storageSettingsForm($form, $form_state, $has_data);
  }


}