The Mysterious Cities of Gold/WMG

Estaban's father was Chinese.

 * Well, it's never made *that* specific, but he's definitely supposed to have come from the Eastern end of the Asian continent. The Portuguese had made it as far East (in European terms) as Japan by the time the story is set. The design of the ship is definitely of Asian origin.

Zia taught Estaban Inca while they were crossing the Atlantic.

 * This disposes of a lot of translation issues in the early part of the series.