Often it doesn't matter, but there are some differences.

'Speak' can refer to the action itself.

I speak Spanish really well and I speak Russian badly.

'Speak' can refer to something more formal than 'talk'.

As a teacher, I spoke to my students in lessons

and then I talked to my friends afterwards.

'Talk' suggests that it's a conversation.

My friends always listen to me, not all my students did.

There are also some expressions where we use

one word rather than the other.

Like: speak out or talk rubbish.

The difference is very small

and often it doesn't matter which one you use

but remember:

Teachers and politicians speak to you,

friends and family talk to you

and you should speak out

if you think someone's talking rubbish.