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.

I agree with Lia. Business life is just one important part of daily life. And we are still dealing with people.

Well, even in business informal expressions are necessary. You usually don't talk with business partners and chefs only about business, but you do small talk. And if you couldn't understand and answers what your partners talking about you're lost in conversation. By the way: Best way to improve English or any other language is to watch movies and read books made and written in this language - by doing so you'll learn all the informal stuff as well. Greets Lia