Entrepreneur.com offers a handy bullet pointed list of tips for conducting business with those who didn't fall directly off the Mayflower. A sample "tip":
Speak more slowly than you normally do, but don't raise your voice because you think the other person can't understand you. Volume doesn't usually increase comprehension. Also, don't speak down to them as if they're children.
This is clearly a lesson the author learned by trial and error after hours of yelling slowly at some unfortunate non-English speaking person who probably had an exponentially higher IQ. "WE'RE HERE TO BUUUUY YOUR COM-PA-NY. AND PAY YOU 83% LESSS THAN MAAAARKET VALUUE."
Avoid slang, buzzwords, idioms, jargon and lingo. These can all be easily misunderstood by those whose primary language isn't your own. Just use simple language until you can get an idea of what level of your language they understand.
"LIQUIDATION PREFERENCE AT TWOOOO AND A HALF TIIIIIMES THE ORIGINAL INVEEEEEEESTMENT..."