Jorge, I doubt very much that you are learning 'formal' English. What you are learning is NEUTRAL English. It's plain, ordinary, everyday language which you could use in any situation, with any kind of person, and in any English-speaking country. Many students make the mistake of thinking that what they are learning at school isn't useful English, or that it is different in some way to what 'real' English speakers use. This is not the case - please don't think that the lyrics of rap songs are representative of how native speakers usually speak! Also, if you read words like 'gonna' 'gotta' 'wanna' and 'em' (instead of 'them') in subtitles and captions, please don't think that these are part of an informal version of English, or that they're a dialect or slang which you need to learn. In fact, they are simply transcriptions of how words sound when we are speaking fast. You don't need to learn them at all. They aren't part of the written language : if you look through the pages here on italki, you'll see that it's only foreign learners who use these words - we native speakers don't generally write them unless we're deliberately imitating sloppy speech.

it is a mix of pronunciation of some words and the use of slangs :) Just like in any other language! Parce.