There was no learning to speak, no going over gibberish until it came. There and then they were given the gift of tongues. Languages take time to develop. Most of us start with a very limited vocabulary. Gradually it develops.
Tongues are like that.
It takes time to develop the gift. Don't give up. They began to speak in tongues. Verses 5 and 6 give the purpose. They inform us that there were men in Jerusalem from all parts of the earth and the Apostles preached to them in their own language.