Answers

I don't think colleges require you to take a foreign language to apply. That being said, I think it's more common nowadays for students to get as much intensive foreign language study as possible. By taking 4 years of HS Spanish, my friend was able to place out of the language requirement at his university. So, you should definitely continue to study, and it's really up to you whether you want to get to more advanced levels of Spanish or get to rudimentary levels of a newer language. Ask yourself what you think you'll benefit more from.

Study as many languages as you want. Study any language that you would like that your high school offers. At the end of the day, your foreign language study choices should reflect your personal goals. If you want to continue studying Spanish, then it may add evidence to the fact that you really are interested in studying "International Studies" in college and becoming a translator. If you study a variety of languages for surface knowledge, then it may add evidence to your claim that you want to get a PhD in Linguistics.

Study for another year in Spanish. This way, you can pass out of any foreign language placement tests when you get to college and then you can have the freedom to choose what language you want to study on the college level.

For applying, colleges don't care. But most colleges have foreign language reqs. So, the more study you get before college, the more prepared you'll be by the time you get there. If I were you, I'd try French. It's very similar to Spanish so it should be a natural complement.