Native English speakers learn English from the day they are born. Grammar rules are taught in primary school. However, there is a big difference between formal written English and spoken English just like it is in your language. People all over the world use slang on a daily basis and it changes very quickly. Slang also varies from one English speaking country to another. The Russian language now has many English terms in it. Am I right? You might find www.urbandictionary.com useful for looking up slang.

How do people speak in your language? It's the same!

When we talk most of the time everyone speaks in perfect grammar, I think that just comes natural like with any language. Maybe they will "break the rules" on purpose to try to be funny or in a funny situation. However in English, even though the grammar is incorrect when you write it down, sometimes it is accepted in oral English. Sorry can't give an example here.