I believe the Puerto Rican government uses both unofficially. Officially they are a US territory, which means de-facto requirement to use English.
According to Puerto Rico's Law 1 of 1993, English and Spanish are the two official languages of Puerto Rico. A previous law, in 1991, had made Spanish the sole official language; Puerto Rico enacted both pieces of legislation itself, as provided for under the Treaty of Paris and relevant US legislation. Furthermore, the Constitution of 1952, enacted by the United States Congress, and ratified by President Truman, provides that all members of the Legislative Assembly of Puerto Rico must speak either English or Spanish. I'm not clear how or where you're finding this de facto requirement to use English, but on a de jure level it's clear that congress has repeatedly, since at least 1903, accepted the use of Spanish as an official language in Puerto Rico.
So Santorum's arguement is completely off the wall, and he could have prevented a gaffe like this if he had just googled the official language of Puerto Rico.