code: def _cache_translation_needs_fallback(instance, language_code, related_name, timeout=cache.default_timeout):
      
      if not appsettings.PARLER_ENABLE_CACHING or not instance.pk or instance._state.adding:
          return
  
      tr_model = instance._parler_meta.get_model_by_related_name(related_name)
      key = get_translation_cache_key(tr_model, instance.pk, language_code)
      cache.set(key, {'__FALLBACK__': True}, timeout=timeout)
docs: Store the fact that a translation doesn't exist, and the fallback should be used.