The language is Spanish, though many Ticos speak English fairly well, especially if they work in tourism.
Here are two links to help you learn the basics
Spanish Dictionary: Type in your Phrase and it will translate it for you
If you are visiting, you won’t have to know how to speak Spanish, but like any country, the effort may be appreciated and you will have a better time.
Oh, and one other thing. If you DO meet anyone, and you will… a hotel clerk, a store employee, waiter, or other person who doesn’t speak your language, don’t talk louder or slower. It won’t help! They are not deaf nor stupid. They just don’t speak your language. They speak theirs.
For emergency, call 911, but they may not speak English.