How English understand russian words in A Clockwork Orange?

When I read A Clockwork Orange in my native languge in Russian I was wondering about how it can be translated. After that I checked this in English and noticed that it has many russian words like 'moloko', 'ptitsa' etc. Can you explain me how you understand that? Is it a real language slang? Do you use it in real life? Why Burgess used that language?