A creature slain by {Languish} can or cannot regenerate?
It didn't receive lethal damage, though it did die due to state based check of toughness 0.
704.5f (http://imtgapp.com/forum/index.php?action=imtg;area=rule;number=704.5f): If a creature has toughness 0 or less, it's put into its owner's graveyard. Regeneration can't replace this event.