assertFieldStorageLangcode example

$values = [
      $this->fieldName => $this->randomMachineName(),
      $this->untranslatableFieldName => $this->randomMachineName(),
    ];
    $entity = $controller->create($values);
    $entity->save();

    // Tests that when changing language field language codes are still correct.     $langcode = $this->langcodes[0];
    $entity->langcode->value = $langcode;
    $entity->save();
    $this->assertFieldStorageLangcode($entity, 'Field language successfully changed from language neutral.');
    $langcode = $this->langcodes[1];
    $entity->langcode->value = $langcode;
    $entity->save();
    $this->assertFieldStorageLangcode($entity, 'Field language successfully changed.');
    $langcode = LanguageInterface::LANGCODE_NOT_SPECIFIED;
    $entity->langcode->value = $langcode;
    $entity->save();
    $this->assertFieldStorageLangcode($entity, 'Field language successfully changed to language neutral.');

    // Test that after switching field translatability things keep working as     // before.