(As a European, I'm not used to think of Spanish as Poor People Language (my grandma was born in Spain!), but I do realize that's the reality in the USA.) So yeah, no Spanish for Fisk.
Not a secret code exactly, but they could maybe manage a quick exchange over Fisk's associates' heads. Over Fisk's, too, if Wesley isn't there to translate.
Or maybe not a secret at all, but they speak Spanish anyway because they want to/Juana misses her roots.
Re: All about OCs // Older Female Mentor OC - 1. (Fisk + languages)
Not a secret code exactly, but they could maybe manage a quick exchange over Fisk's associates' heads. Over Fisk's, too, if Wesley isn't there to translate.
Or maybe not a secret at all, but they speak Spanish anyway because they want to/Juana misses her roots.