NOGO

the Spirit was giving them utterance.
So the Spirit gave them what to say.
Clear, isn't it?

because each one of them was hearing them speak in his own language
Wow!
So speaking in "tongues" means speak several languages at the same time. Each heard what was said in his own language.

... But Paul told them to shut up because nobody understood.

So Paul discovered what we all know today, that is that speaking in tongues is just a myth.

Since the Spirit told them what to say then Paul told the Spirit to shut up.

But how can you go
FROM
Each heard and understood it in his own language
TO
Nobody understood it.

Another mystery!