The people I have offended this way is crazy... it's not meant as an insult, but as the article states "what do you want" is already shocking, so just imagine if anything more direct is asked. :(
If you're looking for advice here, most people will be much more forgiving if you open with something like "I'm afraid this isn't my first language, forgive me if I get things wrong", delivered with a sunny smile, as one of the first things you say. (I'm a Brit; this is intended for speaking to Brits.)