Language is such a melting pot, and it's very likely that English is the biggest melting pot of them all. Of course, I don't really have much experience with many other languages. I know that the Swiss language, spoken, is very region specific, being a mix of German to the north, French to the west. Russian to the northeast, Austrian to the east, and Italian to the south. The Swiss language, written, is German. At least that's how I understand it. I can't speak it. I took German when I was in college, but I could never remember the vocabulary. But I also took a class on Old English, which I found fascinating, but which I also failed.
At any rate, regardless of my ineptitude at languages, I love listening to them, and because I'm good at listening, I can hear similarities between them. As we (humans) mixed and moved around over the centuries, our words also left their mark wherever we went, and as an end result slang took different forms in different regions. I read (or tried to read) a book once where the slang was so unfamiliar and so liberally used, that I lost the content of the story from time to time. As I recall, that writer was somewhere in the south of Ireland, I think.
It's not that slang is a bad thing. Quite the contrary. It adds a flavor and it adds character to one or more characters. You just have to be careful to make sure all your readers can understand it. If you have someone who is from the out back of Timbuktu whose slang is so stiff it's nearly another language, you might want to add in a new friend from somewhere very proper (and yet within reach). That way you can have someone constantly asking for clarification. Might add a little comedy too if misunderstandings can abound. If it's just the odd word, make sure the surrounding content adds definition.
At any rate, don't lose your reader to slang.