Assuming you mean the 4 LCME-accredited schools in Puerto Rico, yes you're expected to be fluent in Spanish, by the time you interview. These are schools funded to train PR-resident doctors; they are not "Caribbean" schools for mainland kids. About 10% of students are from the mainland. Spanish instruction isn't provided.
Edit: I should clarify that: Instruction is in Spanish and in English. Rotations are pretty much pure Spanish. Non-Spanish speaking students aren't accepted into these schools. Maybe 10 years ago this wasn't the case, but it's much more competitive now.