The names are not Spanish, tl, those are italians!
The whole concept of the Twin Kingdom is essentially Italy italy, with the Jillbell Papal family being the Pope and the Roman Curia, while the Sharowa represent other Italian nobility. The names are all distinctively Italian, from Bruno to Giuseppe, with Largo being an Italian word (yes, also a Spanish one with sligthly different meaning) and Lucrezia Broglie probably inspired by Lucrezia Borgia. Even the concept of Northern people migrating into the country under invitation of local powerful families and coming to form a new nobility is how medieval Italy was shaped, it's precisely how of the Norman Kingdom of Sicily was born.
Edit: Mentacato Duchy is probably a play on the word "mantecato" which is a way of making a food (most commonly codfish) creamy through whipping. Also goddamn, with the amount of internal conflicts and secret plots it really couldn't be anywhere but Italy