It seems a little hard to do this and not have some sort of understanding of what is going on at least. Mostly if your native language is not even asian.
I think english can be learned by just listening to it a lot, but it also depends on the region you come from. It's easier for germans to learne english for example, but for asian people it's not so easy.
I think slang helps people come up with their original way os speaking in their regions and gives more individuality to them. I believe it's a great thing to have.
Thing is that for example saying "con permiso" and saying "disculpeme" while you are navigating through a crowd of people in order to get out of the crowd, it's the exact same thing. It also depends on the region or country.
I just noticed that I wrote the title incorrectly lol. Anyone here learned TO speak more than 2 languages very well? Just correcting this so that no one ends up using it to learn.