Most of the presentations I've delivered in the last few years were in English. When I compare the experience of giving a presentation in English versus delivering one in Slovene, it always struck me how much more exhausting it is for me to present in non native language. At the onset of the presentation, everything goes well, but then I become increasingly tired, my tongue starts to twist and I begin to make mistakes. After I make a string of mistakes in pronunciation and grammar, I start focusing more and more on my English and loosing track of the subject matter of my presentation. I watched several videos of myself delivering presentations in English, so I know that my English is not as riddled with mistakes as it feels while talking on the stage. I've asked Damian Conway about this subject while attending his Presentation Aikido tutorial last month at QCon London and he gave me several very useful pieces of advice. First, he said, the trick is not to care about your mistakes (they ain't so big anyway). The audience didn't come to hear your perfect English but because of your expertize. Second, spend a few days speaking extensively in English before giving the talk. This is especially important if you don't use English on a daily basis. Third, you should practice your talk in front of a native (or well versed) English speaker. She can point out frequent mistakes and the use of awkward phrases. Fourth, the amount of practice required to give a good talk in non native language is double what you would need in your native tongue. Finally, being a non native speaker is not always a disadvantage. Quite often native English speakers use complicated words and less known idioms, thus making their talk less comprehensible to a typical audience of non native English speakers. And I've experienced it myself several times, that a native speaker with a strong local dialect is often harder to understand than even a Chinese who commands no knowledge of English but has learned his talk by heart.
What's your experience with presentations delivered by non-native speakers? Either from the point of view of a speaker or audience.