There are only too many slangs to learn for non-locals or non native speakers. I do not know how other English learners regard. My way is not intended to involve in these slangs too much. To learn "natural" regular expression at first can make you feel more confident. I agree with peachey's saying:

"On the other hand... I often see students trying to use slang too early because they believe it will make them sound more natural. Don't do this! It makes your standard English sound awful and it makes you seem like a "wannabe". There's nothing wrong with using regular English."

I teach both when I teach. Even in my university classes. I would say most is formal but I bring up common terms used regularlly. After all there is so much that most people would not understand. I once had a whole class on Sports terms as they are so prevalent in our language and some are even country specific like Stick Wicket is not an American thing though I think many understand the term here. Also terms like getting to first base is from baseball so a Brit might not understand.

Informal things are so muhc a part of our regular speech that even trying to teach everything you can think of you will forget. So if you have not been around a language your whole life your might not get it. Even as a native you may come upon things you dont understand because of regional difernces whithin your own country. When I do know those I also try to teach them. Like in the US some places use coke (the brand but used for any soda),cola, soda, or just pop (short for soda pop) though soft drink would be more formal but no one says I will get a soft drink though it would be correct to do so. In this case if I was teaching that to my class I would say these are your options I tend to use soda or cola and yes soft drink is an option but no one uses it so you shouldnt.

I understand - you don't want to sound like robots! :)

Like Chris, I like to give "colloquial" or "localised" options if I think the class will understand it well enough. That means understanding the difference between standard English and what an Australian may say amongst old friends, for example. Some expressions are different, or even unknown, among English-speaking countries, so it's helpful to know that Brits say one thing, folk from the USA say something else, and it can even be different within just one country.

On the other hand... I often see students trying to use slang too early because they believe it will make them sound more natural. Don't do this! It makes your standard English sound awful and it makes you seem like a "wannabe". There's nothing wrong with using regular English.

Between native speakers, we actually use regular English (ie. what you are learning now) when we first get to know each other. We really don't use any slang until we think it's appropriate and understood. This means listening to each other.