I have been overcoming the PAF name limitations for years and have tried different ways to input the names so that they make sense and are handled properly in a database (sorting, searching, printing).
When it comes to my Spanish names, I have ended up doing the following as the best compromise:
Since each person has at least one first name and at least two surnames, then I use the first name and middle name fields as expected but put both last names in one field, no hyphen, just a space.
Ricardo Diaz Soto is inputted as FN: Ricardo LN: Diaz Soto
This facilitates entering the siblings for RDS but, more importantly, distinguishes him from Ricardo Diaz-Soto, which can be a surname in itself, especially for US emigres. Having had to work do work arounds in other programs and having had to retype/correct many of my names many times because of programming limitations, I am not exactly eager to do it again but if there is an international standard that differs from my method, I don't know it.