> I notice that if I elicit a similar error via "osascript," it also remains in English. Could this be related to the fact that I'm a faceless background application - or is it something simpler than that?
Does your program have a Spanish.lproj? If not, try this, just as an experiment: copy, link, or alias Spanish.lproj to your existing English.lproj.
I think you'll see the messages being localized, now that the OS thinks you support the language.