The coming of the Holy Spirit is a mission shaping and mission enabling event. With the coming of the Spirit, what seemed like mission impossible for the disciples on their own, becomes mission possible. The Spirit straight away enables, equips and empowers them to burst forth in praise of God for the salvation he has won for us in Jesus. They just can't keep their praise in. And as they joyful exuberant praise flows the Spirit enables them to praise God in other languages drawing a crowd to hear the good news.
The crowd come running and Luke stresses its multinational nature. This isn't a low key, gradual start to the mission. Filled by the Spirit who Jesus has poured out the first day of mission sees a picture of the reach of the gospel to the ends of the earth with the regions represented in the languages present.
But this spectacular languages show is just opening the door, it is just providing an opportunity, the disciples need to speak. The crowd has been gathered, they are amazed, they are full of questions, they want to know what is going on. But the crowd need someone to explain for them, they need the disciples to explain the events in the light of Jesus.
I wonder often if we find evangelism so scary because we forget that God has given us the Spirit to enable us to witness to Jesus too. We're not alone, we're not without help. But we need to be bold enough to take the opportunities that the Spirit opens up for us, all be it in less spectacular ways than on the first Pentecost. Mission is possible because God is faithful.