The first variety seemed to be without need for interpretation because all those who heard it heard their own language being spoken.
And how is it that we hear, each in our own language in which we were born?
The second one the church is warned not to utter without interpretation. Paul said in 1 Corinthians 14:28 (NKJV):
But if there is no interpreter, let him keep silent in church, and let him speak to himself and to God.
And in 14:23:
Therefore if the whole church comes together in one place, and all speak with tongues, and there come in those who are uninformed or un-believers, will they not say that you are out of your mind?
What makes the initial event different from the subsequent?