There is only a subtle difference between them and most people would not bother with the difference. 'do you speak English' just asks if one is able to speak English 'can you speak english' could be used if you knew someone else does speak English and you want to use that language for communication. But either can be used to find out if a person does speak english. So very subtle and interchangeable in common usage.

"Do you speak English" means: "Do you have the ability to speak English or do you know English ?" But "Can you speak English" is a request that if you have the ability,can you speak this language. In the other cases,both of them are equal.

for example.i am from china.i can speak english because i learn it but i dont speak english in my life i can but i dont speak i hope my answer can help you

It would be like me asking you, Do you speak Korean? & Can you would be a ability. For example: -I can speak English and Spanish.