Speak and talk are interchangeable. However, one or the other is preferred in certain situations.
1.Generally, talk is more informal, speak is more formal and more serious:
Could I talk to you about my birthday after lunch?
I need to speak to my son before he gets himself into serious trouble.
John is going to talk to us about Youtube cat videos.
Jane is going to speak about the recent disintegration of the economy.
2.People speak languages.
He speaks many languages fluently.
He needs to learn to speak French before his holiday.
3.People speak on the telephone.
Hello, can I speak to Mike please? Hi, Mike speaking.
4.Nonsense and rubbish.
Don’t talk nonsense/rubbish.