This was at a time when not many people spoke English ( those who had the chance to learn a second language usually studied French) and they figured audiences would have an easier time understanding Spanish names. If the name to translate didn't have a Spanish equivalent they just picked a different one.

I don't understand the Kira/Nora thing, though, because Nora isn't a Spanish name at all as far as I know. It could be an attempt at making it easier to pronounce, but Kira is easy enough, I think. Is there any wordplay or anagram about the name in the book? In Harry Potter and the Chamber of Secrets they had to change Voldemort's name to Tom Sorvolo Ryddle so it could spell "soy Lord Voldemort".