No, a thong can be either in American English. The word simply means a strip of leather or something similar, so it can be used by synecdoche to refer to a garment including a thong, whether a sandal held on the foot by thongs or a bikini bottom held on the body by thongs or strings. But what Americans call a garter belt, the British call suspenders; what Americans call suspenders, the British call braces; and what Americans call braces, the British call a mouth-brace. Meanwhile, what the British call trousers, Americans call pants, and what the British call pants, Americans call panties. So if an American man walks into a British clothing store and asks to buy pants and suspenders, people will look at him funny. Except Firefly/Serenity was very unrealistic about that. It wouldn't be modern English interspersed with modern Chinese. It would be a single language that was a creole of both. Of course, that wouldn't have been comprehensible to most modern-day viewers. As a native language, yes. But according to another column on the chart, if you include people who speak it as a second or other language, it's in the #2 slot, probably first if you include people with a partial knowledge.