The first sentence is definite - you are speaking English with a Korean accent.
The second implies you are speaking English, since the sentence is written in English. However, logically I couldn't say for sure that it is English you speak. It could be any language.
If the second sentence was "I speak English in a Korean accent.", then the meaning between the two would be the same. I have heard both forms, so I am not sure if one is more correct than the other. However, I don't know of any English speaker that wouldn't understand both sentences in the same way.