بسم الله الرحمن الرحيم
Yes Mr.Faros, using the slang is not recommended , but you can't apply the formal language all the time,,,what I mean is you can use the formal language at certain places,such as, school, doing business, in an important speech to the public. writing a report or a letter to an important person. I'm not against learning the formal language....but we should differentiate between the formal, informal and the slang . once we know the difference , we will be able to know when we should use each of them. Although the slang is the most common used language, but it's not accepted at the shcools,universities,meetings etc.