Swearing in Chinese is stupid - people always swear in their native language.
Well that's simply not true at all. One of the things that's always bothered me in fiction is that the only things people say in their native languages (when they're foreigners in an English-speaking group), are the very phrases that you pick up quickest living in a foreign country - greetings, simple exclamations, yeses, noes and swearing. I often swear in Czech, even though my spoken Czech is terrible.
What's more, if you look at countries heavily saturated with English-speaking media (the ones that don't dub, that is), you'll often find that people use 'fuck', 'shit' and the like quite a lot. Try watching any Dutch-language television.