We've all seen it. Some poor tourist getting screamed at, louder and louder, because they can't understand what a native is saying. Other times they are spoken to as if they are mentally deficient children. This is no way to treat a guest of your country! They're foreign not deaf!
I speak a few foreign languages myself, and I've noticed some things that people do that make it much harder for me to understand them. Here are some ways you can make it easier for someone who's not completely fluent in your language to understand what the heck you're trying to say:
Speak clearly- Open your mouth! One of the most frustrating things on Earth is trying to understand people who mumble, and it's a hundred times worse when it's not in your own language. Don't mumble. Enunciate, but not exaggeratedly. Also, talk loud enough so that they can easily hear you; don't whisper, and don't yell. The louder you get, the more nervous they will become and it's not very easy to think when you're nervous. Speak slowly- When I speak Spanish to customers at work, I often have to ask them to slow down 3 or 4 times. It's not that I don't speak Spanish well; I just can't process it that fast out of the blue. So, speak slightly slower than you would to a native speaker, but don't talk extremely slow aaas thiiis maaakes youuu soound liiike aaan aaaaaaaasshole. Avoid using slang- I was out once with my friend and his fiancée's mother, who was from Lebanon. She was fluent in Arabic, Farsi, and French, but had very limited English skills. At the supermarket, she didn't quite understand how to use the debit card reader, so the clerk tried to explain it to her thus: "First slide your card in the slot, then press the 'debit' doodad, then enter the magic number." The poor woman had no idea what he was talking about. This is a big one. When people learn a language in school, they don't usually learn much slang, so speak to them formally, or with the language you'd use to write a paper for school, minus the large words and idioms. Be patient!- This is the most important rule of them all. Having to speak a language that is not your own can be very intimidating. Give the listener a break; if they don't understand something you say, try saying it in a different way. Smile, speak in a pleasant tone. Try to put them at ease. If they're relaxed, it will be much easier for them grasp what you're saying. Just imagine yourself in their place, how would you want someone to speak to you?
These rules seem like common sense, but many times monoglots just don't have any common sense when it comes to this sort of thing. Never, ever, EVER assume that someone is stupid just because they don't speak your language. Think before you open your mouth, and you will save yourself and the other person a lot of trouble.
From Albert Herring: If you're an anglophone talking to romance language speakers, "long words" are likely to be easier than short ones because of common etymologies ("arrive" is more obvious than "get to" to a French or Italian speaker). Avoid using phrasal verbs. Get up, get on, get on with, get off with, get into, set up, put up, etc. | 
