Pass langcode from view elements.

Closes #3492350

src/Plugin/Field/FieldFormatter/BaseFormatter.php

@@ -16,7 +16,7 @@ use Symfony\Component\DependencyInjection\ContainerInterface;
 /**
  * The base formatter for custom_field.
  */
-abstract class BaseFormatter extends FormatterBase {
+abstract class BaseFormatter extends FormatterBase implements BaseFormatterInterface {
 
   /**
    * The custom field type manager.
@@ -224,7 +224,7 @@ abstract class BaseFormatter extends FormatterBase {
     $elements = [];
 
     foreach ($items as $delta => $item) {
-      $elements[$delta] = $this->viewValue($item);
+      $elements[$delta] = $this->viewValue($item, $langcode);
     }
 
     return $elements;
@@ -268,15 +268,9 @@ abstract class BaseFormatter extends FormatterBase {
   }
 
   /**
-   * Generate the output appropriate for one field item.
-   *
-   * @param \Drupal\Core\Field\FieldItemInterface $item
-   *   One field item.
-   *
-   * @return array
-   *   The textual output generated.
+   * {@inheritdoc}
    */
-  protected function viewValue(FieldItemInterface $item): array {
+  public function viewValue(FieldItemInterface $item, string $langcode): array {
     return [];
   }