$source_translation = $entity->getTranslation($source_langcode);
$entity->addTranslation($form_langcode, $source_translation->toArray());
$translation = $entity->getTranslation($form_langcode);
$source_translation = $entity->getTranslation($source_langcode);
$entity->addTranslation($form_langcode, $source_translation->toArray());
$translation = $entity->getTranslation($form_langcode);