For them not to slow down and pick the simple words for you, the secret is you have to speak with their speed and accent. Eventually they won't even feel your language skill is different from theirs any more. Try a couple of slang words you just learned from them, they will laugh at first, but now you are with them.
The cubical culture is unique. My office has open cubics, just turn my chair around, I can join the conversation. Lots of jokes are insider jokes to the group, not necessary slang or idioms. Pay a little more attention to what's happening around you, you should be able to catch up. Don't feel intimidated to ask: "What's so funny?" every once a while. Depend on your working environment, you can walk to other people's cubic to chat with them as well. People in my office do that all the time.