Slater communicates with the kitchen staff in fluent Spanish, but in an episode of Saved by the Bell: The College Years (1993), Slater has a hard time understanding even basic Spanish.
During the Thornhill party, Screech hauls in a pile of heavy coats belonging to the guests. Why would anyone have worn a winter coat to a summer party on a California beach?
Stacey says that they can't have Jenny Richter's party out on the beach anymore because it's all dug up with holes, but there's an outdoor cafe with a wooden deck that they could've use for the party.
Slater translates for Roberto, but it is clear from his correcting Slater's translation that he understands and can speak the language.