Cambridge Dictionary gave the following rules that will help us learn how to use “speak” and “talk”
The verbs speak and talk both generally mean ‘say words’, but there are some small differences in how they are used. Speak is more formal than talk.
(a.) Speak- We usually use speak for more formal presentations and lectures, and talk for more informal ones.
Ex: Dr Graham Foster will speak about the history of the region.
-When we refer to languages, we use speak.
Ex: How many languages do you speak?
–Speak usually only focuses on the person who is producing the words:
Ex. He spoke about the importance of taking exercise and having a good diet.
(b) Talk focuses on a speaker and at least one listener, and can mean ‘have a conversation’.
Ex. I hope I can meet you to talk about my plans for the company.
-We use talk to focus on conversation.
Ex. They can talk and get to know each other over dinner.