i am an american, english is my mother tounge of course, i would have to say the trick to learning to speak everyday english is informality, unless we americans are doing business we pretty much don't use any formalities. we are lazy speakers, that's why we have slang, we're lazy people when it comes to language, or at least most of us are.
Why is it that I always read the same question and the same answers. How can you say English it's the trickiest language, or the easiest one? or is that lazyness is the cause for slang in English?

Everyone thinks that his/her own language is the trickiest, the most difficult , the one with more slang... What happens is that as an English speaker, you know a lot more than non native speakers about its nuances, its slang, its tricky features. But you don't know so deeply other languages, and then you think others are more simple. Or maybe, as you know yours since childhood, you think others are more difficult.
But all in all, more or less all languages have the same complexity, otherwise there would be cultures in which children would acquire their spoken languages before others, or after others. And that's not the case. Writing is another question.

Maybe, instead of comparing English to German, Russian, Arabic, Japanese and Spanish (5 out of 5000 languages, 0.001 > 0.1% of all languages, that's nothing), we could compare English to some 500 languages (10%) and then take some conclusions, don't you think?

Have you heard about Ergativity in Basque or Caucasian languages? Have you heard about click languages, such as Xhosa, to talk about difficult pronounciations? And what about those consonant clusters of Georgian?
Or the 9 (are they really 9?) tones of Cantonese?

One last thing, if lazyness causes slang, then all language speakers are lazy. Do you think Spanish, for example, has no slang? And yet, we have dozens of verbal forms.