Hi Christiaan,
My fiancée is Brazilian and is still learning English. (Although she does a good job!) Even though I have known her for more than a year, I still don't know what words she will know and what words she won't. She often knows words I would think are hard and doesn't know words I would think are easy. She still has trouble with "to run out [of something]" even though I've explained it a couple times. But she has no problem with words such as archive, endocrine, spermatozoa, epithelium, onomatopoeia, and dissonance. I don't even know with my own native-English-speaking friends when they will stumble on a word that I assume everyone knows (the most recent one was 'smarmy').
Generally, when I write, I try to use the perfect word to convey the meaning I intend. Sometimes that word is a little more uncommon than other times. When I am aware that my audience may not be familiar with the word, I still use it, but I also throw in a brief explanation. I try to make it all fit together without sounding as though I am one the one hand intentionally using hard words, or on the other hand as if I am lecturing and explaining what the person might well know extremely well!
Other times, when I don't know there is a need for me to throw in the extra explanatory phrase, people have to ask for clarification or go look something up in the dictionary. If my girlfriend has to have lots of patience with me when I use such words, the other side of the coin is that I have to have lots of patience with her when she has trouble conveying her meaning to me. The reasons are different (I use hard words, and she doesn't know the words), but the communication problem is the same.
Oh... also, half the time she has to be patient with me for explaining things she already understands or I've already explained before. In the end it's all about grace and give-and-take.
Honestly, I would say that if someone is using somewhat hard words, you could take it as a compliment that they have a high opinion of your intelligence and comprehension... as several people in the thread have suggested, deliberately using small words to a non-native English speaker is often a sign of contempt.
E²