When we listen to a foreign speaker, we almost always hear an accent. People can live in the US for decades and still have an accent. Often it is a "charming accent." But, it makes them just a little hard to understand. You can compensate for your own accent by speaking clear, direct, ordinary "classroom English."
When foreign speakers use idioms or slang, often they don't get them quite right. They don't use them quite right. We are not expecting to hear them. It doesn't make them sound "more native," it just makes it harder to understand them.
Once, I was traveling with a friend from Puerto Rico. He spoke perfect English, but with a slight accent. We were tying some things to the roof of a car. It was important that they be well secured. He tightened the knots. He tested by trying to shake them. I said, "Is it OK?" And he said:
"It's not going anywhere."
And for a split second, I couldn't figure out what he meant. Why not? Because a native speaker would have said
"That's not going anywhere."
The stress would be on a different syllable, and the native speaker would say "that" instead of "it."
A slang expression or an idiom with the wrong rhythm or intonation is like a mispronounced word. It's hard to understand.
Worse yet, when it comes to slightly "bad language," there are cultural rules for when, how, and where you can use bad words. You really have to live in a country for a long time and absorb the culture to know them. People in movies or YouTube videos use "the f-word" a lot, but don't be fooled. It's drama. It's not real life.
Learn slang to understand it. It's really best not to use it. But don't worry about it too much. People will forgive the blunders of a foreign speaker, just as they forgive the blunders of children when they use bad words at the wrong time.
If you want to sound more native, just learn the standard language well. :)
To be honest, if I (a native English speaker) used words like dope, sick, lit and savage with my (native English-speaking) friends, they would laugh at me, or at least give me a funny look. Why? Because it's the wrong context, for a start. I've never used that kind of slang with them before so they'd think I made a joke... probably right. Also, I'm probably too old to be using such words because I grew up with different slang. Even the words can be too old! I've already seen "sick" and "lit" replaced with "ill" and "fire".
If you want a more casual feel to your English, then talk and listen to your real friends. If they use slang with you, then you can use it with them. Simple. You will probably build up your own slang vocabulary with them, and that's exactly how slang works: you have a common understanding with a small group of people.
Don't take youtube, movies, TV shows and so on too seriously, because it's all edited for our entertainment anyway. ;)
Slang is tough. If you don't pronounce it exactly right or you use it in the wrong situation because you don't really understand a shade of meaning -- you are going to stick out and sound awful. I understand you want to use it, especially as English is full of slang and phasal verbs but it is hard to recommend its use to any but advanced students.
Curses too, not recommended :-)
No, using slang doesn't make you sound more native, in fact it does quite the opposite. This was discussed, and Jerry heard it, and I mentioned that slang is very nuanced and varied among native speakers and you really need to have a handle on how to use it effectively. Unless you speak accent-free or near-accent free English and know the language in and out, avoid it. You don't need it anyway. And many natives don't use it at all. There was and still is a TV comedy show in the US, Saturday Night Live, and a popular comedy skit played back in 1978 by Steve Martin and another, was called "Two Wild and Crazy Guys" , and it involved two brothers from the former Czechoslovakia, living in New York City and speaking with heavy accents. The whole joke centered on the heavy accents of the two brothers and their misuse of American slang, and how funny it was. It went over big and very was funny. Even when the slag used was correct, like "Hey, let's go pick up some hot American chicks tonight" it still sounded funny only because of the heavy accents. So, anyone who has a foreign accent who uses slang in English is actually comical sounding. That's probably the best reason to avoid it, let alone the fact that it's easy to make mistakes with and use incorrectly.