Does Hardened Scales interact with the undying mechanic?
Yes, Young Wolf would get 2 +1/+1 counters. The reminder text for Undying says


  When this creature dies, if it had no +1/+1 counters on it, return it to the battlefield under its owner's control with a +1/+1 counter on it.


A ruling on the Hardened Scales Gatherer page says


  “Placed on a creature you control” includes that creature entering the battlefield with +1/+1 counters on it. If a creature would enter the battlefield with a number of +1/+1 counters on it while you control Hardened Scales, it enters with that many counters plus one.


When Young Wolf returns to the battlefield from the graveyard, it enters the battlefield with a +1/+1 counter, so Hardened Scales makes it enter with 2 instead.