Their "concern" is one of those mean-girls thinly veiled shade type of things, IMO, and here's why. If there was a translator nearby, would they be perfectly okay saying the same exact thing to those girls' faces? I think NOT. It's just mean-spirited and low-key shady, as if we've not had pageant winners who spoke not a single word of English! It doesn't make those girls look good, or increase their chances of winning. They were just being petty.
Those girls were being ludicrous, and that "...well, she grew up half-Chinese in Australia..." excuse is a piss-poor one.
I speak 3 languages, English aside. And I've done this thing where I'm in public, and act like I don't speak English and the stuff people say about me or what I'm doing is definitely NOT anything they'd repeat in my face if they knew i understood what they're saying.
My favorite movie line is from "A Walk In The Clouds" - "...just because I speak with an accent doesn't mean I THINK with an accent!"