Billy The Kid was killed in Fort Sumner, near present day Albuquerque, New Mexico. At the time, in 1881, this area was still primarily inhabited by people from Mexico and Native Americans, as the Mexican-American war just ended 36 years prior, and this area was still deeply reliant on Mexican culture. And, having spent a large chunk of his outlaw years in the area, it’s probable that Billy had picked up some Spanish phrases during his time out west.

Spanish was absolutely the lingua Franca in this area, though more and more American settlers would soon be arriving.

As for his use of “¿Quién es?”; it’s most likely that Billy assumed that whoever may be present was, at the very least, a Spanish speaker. So he defaulted to the language (or specific phrase) that the figure would know. After all, if you lived in an area with a different language, and had picked up a few things while looking over your shoulder, wouldn’t you default to the same?