So am I.

Except for the Birth Certificate distinction what is the difference?

I mean it honestly. I really don't understand what that formal distinction you mentioned is. It's not the same as a British person being called Marie-Claire on Anne-Marie, those Spanish examples I gave really are two names (most people who have ever known my mother have no idea that Josefa is her first name, she never uses it)

EDIT- got called away before I could add:
A country which truly has no concept of a middle name (certainly not formally) is Japan. Only found out how inflexible this was when we got our daughter who was already registered with two forenames in Britain registered with the Japanese Embassy. Her two given names got mashed together into a ghastly single unit. And we'd been so mindful of picking out names that would sound fine pronounced in English, Spanish or Japanese