speak and talk can be used in the same sentence in a lot of situations, for example,
I will speak to John
I will talk to John
But there are also situations where we can only use one. These are called collocations where a native speaker thinks the incorrect use sounds wrong.
I am learning to speak English - sounds correct.
I am learning to talk English - does not sound correct.
Let's speak things over - does not sound correct.
Let's talk things over - sounds correct.