I'd like to know  enough slang for understanding books and films. But I don't use any slang, while I am chatting. 

Do you think that people who are not native English speakers use many slang words while they are chatting? 

As a native English speaker and teacher, I am very, very aware of any slang I use.  I often communicate with non-native English speakers, and realise that using my own slang (or anyone elses') would be a huge problem for them.  I'd only drop back into my accent and slang when I'm completely sure the people I talk to will understand it.

When I hear a learner use slang, I'm sorry to say most of the time it sounds very unnatural. This comes down to not knowing when, where or with whom to use it.  Standard English will always be understood, and using slang just to show off is tacky.  I do teach slang, but always with advice on exactly how to use it.

With my closest and oldest friends, we have our own slang of course. That is what I use most often. ;)

I think it's good for a non-native speaker to know a little slang but not much. I do speak with some slang but it all depends to whom I'm speaking. I would never use any slang when speaking to a non-native speaker except when I want to "blow their mind" a little. <img alt="" src="http://www.italki.com/fckeditor/editor/images/smiley/msn/wink_smile.gif" />

As a native English speaker, I try hard not to use slang words. When learning other languages, I don't use them. With my obvious American accent, using slang just makes me sound like I"m trying too hard to fit in.