Any tips on solving monoalphabetic ciphers?
Also, can you get a good score without a team member who knows Spanish?
Keep an eye out for common words. Frequently appearing three-letter words are commonly "the" or "and", so trying to figure those out can be a good starting point. Single-letter words are really nice since they're almost always "a" or "I" (could technically be "O' I guess if it's a poem or something, but that's rare).
Also keep in mind what the most frequent letters in the alphabet are. If you see a letter appearing frequently, it's likely to be E T A O I N or S, which are some of the most common letters. It's probably not Q X J or Z, haha.
According to the rules, "no more than one" puzzle can be in Spanish for Regionals or Invitationals, so that's likely a pretty small portion of your score, if it appears at all. For States, I think it's "at least one" so that becomes much more important, but I would imagine that the question itself is a bit easier to compensate for the fact that it's in a Spanish. I guess if you really practice, you could probably do reasonably well even without knowing Spanish as long as you know the rules of the language. In the end, I doubt it's going to be a killer, though I suppose that might be what separates the very top teams at States and Nationals.