From El cuaderno de Maya:
Una mujer gruesa, de risa contagiosa, saludó a Manuel Arias con un beso en la mejilla y a mí me observó un poco desconcertada antes de decidirse a besarme también.
¿Americana? -le preguntó a Manuel.
¿No se nota? -dijo él.
¿Y qué le pasó en la cabeza? -agregó ella, señalando mi pelo teñido.
Nací así -le informé, picada.
¡La gringuita habla cristiano! -exclamó ella, encantada.
Every translation in context of the phrase is "speaks plain English." Was the language translated from Spanish to English by someone because even in translating to 3rd languages like French still gives "English"?
The phrase only makes sense as Spanish in the example above as they are in Chile, and Wikipedia's article on names given to the Spanish Language
During the presence of Moors in Hispania, Spanish was sometimes given the name cristiano ("Christian") to distinguish it from the Arabic and Hebrew languages...The expression Hábleme en cristiano "talk to me in Christian", uttered to people not speaking Spanish at the moment, is used in opposition of the other languages of Spain, which is felt as annoying by them.
Later wikipedia seems to indicate it may mean whatever language is more clear to the speaker:
"Háblame en cristiano" is also a phrase used to ask for clarification in a conversation, when the topic of the discussion is not clear or is vaguely hinted at by one of the speakers.