When to speak your native language

The 2003 film Shanghai Knights is a worthy sequel to Shanghai Noon. This even though the cleverly choreographed action sequences do start to feel a little routine, and it gradually gets very tiresome to see all the Forrest Gump-style manufactured prescience. But the nit I am picking here today concerns the matter of when characters speak their native language (and consequently there are subtitles on the screen).

When Roy (Owen Wilson) meets Chon Wang's sister Lin (Fann Wong) in a London prison, she and Wang (Jackie Chan) exchange a few words in Chinese, prompting Roy to practically command that they don't talk about him in their native tongue. Out of courtesy, they don't talk about him in Chinese any further, regardless of whether or not he is present.

Roy falls in love with Lin and asks Wang to put in a good word for him. Wang talks to Lin in private, and for no good reason, they talk about Roy in English, not Chinese! They might not know that Roy is eavesdropping (and it does seem somewhat improbable that he can hear anything clearly), but if they suspect it, would it not make sense for them to have this conversation in Chinese?

Also, how exactly is it that Lin and Wang learned English? For someone who hasn't been in an English-speaking country all that long, Lin seems to have a stronger command of the English language than her brother who has spent so much time in the American West.

Of course I do know the reason that that particular conversation is in English rather than Chinese: screenwriter's necessity. The screenwriters wanted a reason for Roy to get mad at Wang over something overheard, to parallel Wang getting mad at Roy over something overheard in the first movie. Dialogue like "Loy悪男LoyBaloney非常不誠実" might just not have quite the same impact on Roy's ego—in fact, that would have allowed him to hear what he wanted to hear, and in any case Lin does seem to like Roy back.