Talk or Speak?

Most of the time you can use either ‘talk’ or ‘speak’ to mean the same thing. You tend to use ‘talk’ for informal conversation:
She likes to talk to her friends on the phone.

And speak is generally more formal and used for one-sided communications.
You should speak clearly when giving a lecture.

You always say ‘speak a language’ and never ‘talk a language’. 

There are a number of fixed expressions that only use ‘talk’ or ‘speak’, such as ‘speak your mind’, which means to say what you think or ‘talking to a brick wall’, which means someone won’t listen to what you are saying.

And there are a number of phrasal verbs that have distinct differences such as ‘speak up’, which is to increase the volume of your voice and ‘talk up’, which means to promote.