Hispanic surnames would, in the US, be initialized just like Irish - “O’Brien”, for example, would be “O”, and “de la Vega” would be “D”. In American English, “O’”, “Mc’”, “Mac’”, “de”, “del”, “de la”, “von”, “van”, “van de”, etc, are all part of the surname. In reality, they are just patronymal or locational prefixes and should (I think) be ignored when initializing or alphabetizing, but I’m not in charge of that sort of thing.
I had a Chilean friend inscribe her initials on a personal bible, she shortened Maria Teresa Sanchez Aguilar to Maria T. Sanchez A.