For the love of all that’s good and holy: when writing a character of an ethnicity not your own, please please please get someone who is one hundred percent fluent in that language to check and double check your foreign language expressions!
Yes, if you are writing for the US market it’s likely that a good chunk of your readership will skip over those and be happy, but for those who actually speak that other language it is grating and annoying and-I’m sure unintentionally but all the same-offensive.
It is not that hard to find someone who actually speaks other languages like a native (or who is indeed a native). It is not.
What brings this on, you ask? Well, one too many pseudo-Latino character using pseudo-Spanish expressions that no Spanish speaking person would use.
“¿De cierto?” does NOT mean “are you sure?” (If I am very very charitable, I can pretend that it almost means “in truth” but even in that case it is not grammatically correct. In any case, most Spanish speakers would say, “¿De verdad?” instead.)
If you want your character to say “are you sure?” s/he should say, “¿estás seguro(a)?” (“seguro” when asking a male; “segura” when asking a female).
The person so asked would never NEVER, under any circumstances answer, “Muy cierto.” Because “muy cierto” means-depending on context-either “very true” or “that is true” (as in, “I agree with what you just said”)
The correct answer to “¿estás seguro(a)?” (are you sure?) is “sí, estoy muy seguro(a)”
Spanish, like French, also has that little thing with the formal and informal speech. Siblings would never ever ever use formal speech with each other. A young woman would not say to her sister, “Es su vida.” She would say, “Es tu vida”
But wait, you may ask, why is it offensive?
Because it often feels as if the trouble of finding someone to make sure these things didn’t make it through was not worth the author’s time, and no one likes thinking they are not worth the time to get it right.
Nota bene: I do not know why an author didn’t get it right; I do not know what the time constraints were; I do not know any of that, so I am not saying that these are the author’s motivations. What I am saying is: this is how it feels to me.