Well CNN English is different because those people need to speak very clearly. However, accents can make things very difficult to understand if you haven't spend a few years in that region. Don't worry about the slangs that much since in a professional environment, they are not used that often if not at all. If you are not sure, ask what the other person meant.
PS: There was an accident long time ago between KLM and some other airline in Africa because of communication break down. The pilot thought the ATC said he had clearance to start the take off roll while the ATC really meant was to hold short. End up head on collision with everyone dead.