This may sound like a silly question. How can you not know which language your presentation is going to be in?
English is the language that's spoken the most at conferences (not limited to tech conferences) these days. But what about a conference in a country which does not have English as its main language? Which language would you use? Especially if you're a native of that country?
If you're only doing presentations in English in English-speaking countries then you can probably stop reading now.
