Extracts the translatatable data structure from the given field.
\Drupal\Core\Field\FieldItemListInterface $field: The field object.
array $data An array of elements where each element has the following keys:
\Drupal\tmgmt_content\Plugin\tmgmt\Source\ContentEntitySource::extractTranslatableData()
public function extractTranslatableData(FieldItemListInterface $field);