What should we learn, English as a Lingua franca or "formal" english?

Learning english is very important because it is the most useful language in the world. But, which is better? lingua franca or formal english? Almost everyone in the world speaks a little of english, this is called lingua franca, which means speaking just with the ability of communicate with others in very common situations. Learning english is easy, but speaking like a native is very difficult, that's why  many people speaks english only as lingua franca. Nowdays formal english in not used as lingua franca, even though books, web pages and many other things are in english. Speaking formal english is more complex , it's not common as lingua franca but is better to learn it because it helps you to understand fully your english. So, I think its better to speak formal english, because when you are speaking it, yo are teaching the speakers around you, it's easier to get access to knowledge and the principal one: you are communicating with others, expresing yourself in a clear and correct way.