Speak and talk have similar meanings. They suggest that someone is using his voice, or that two or more people are having a discussion.

We can say:
speak to somebody
talk to somebody
speak to somebody about something
talk to somebody about something

How old were you when you learned to speak?

What are you talking about?

Who were you speaking to on the phone?

Who were you talking to on the phone?

I was speaking to Mark about cricket.

But we say:

speak a language NOT talk a language
talk nonsense NOT speak nonsense.

Speak to and talk to are used more often than speak with and talk with.

He speaks four languages.
Stop talking nonsense!
I was talking to Tom yesterday.