I think slang is less important than informal expressions. However, a lot of Americans, English people or Canadians will use coquailisms to describe a thought, expression or feeling. A bad joke example would be the expression 'Come on man! Your bustin' my Balls!' when negoiating.
I think thought that when dealing in International Business English that the native speaker of English should avoid expressions to avoid confusion. When you begin to use slang in a business deal, it makes the business deal much less formal and often lowers your position when bargaining I feel.
If you are neogiating a divorce or large contract between major companies for example, they will rarely say "Yo dude! Lets get this deal done already snizzle! I got a hoe I got to meet in 15." the other people in the negoiation will have to lose a bit of respect for you.