Essentially vetarnie, my point is that one gains little or nothing by using slang.
You can "sound" familiar to other people, while at the same time, appearing "phony" to nearly everyone.
Merely by speaking with and or reading in the language studies you do, you will normally acquire familiarity with common slang. That will happen. Some of it you can actually use.
Unfortunately, without knowing the subtle intricacies of an entire culture, you can end up committing a grave insult to other people by employing slang at the wrong time, in the wrong place, and in the wrong manner.
Besides which, I can share with you from my own experience in the Italki forums, I have watched foreign students of English ask questions about the meaning of a slang term and when and how it can be used. It becomes so complicated in the explanations that studying the Rules---of---Grammar looks like child's play by comparison.
In addition to which, the FREE use of language as an entertainment, can barely be understood by foreign students. I can make up a slang term right here, or cite an odd one, and show you its use in American dialogue and you may well never be able to understand it. You would be left thinking; "...but I don't get it."
If I repeated the line from a comedy movie, could you really understand how I would use it?
If I said; "Goonga-La--Goonga!" in my normal speech with you, would you know how and when to use that to "fit---in" with the Locals?
A person could use so much slang, that all of their communication is cryptic and mysterious, not actually related to English at all. You can see an example of this in the movie "Airplane" the comedy, where one Caucasian woman proposes to translate the "Jive" dialogue spoken by 2 Black passengers.