A very particular Spanish
During your trip to Costa Rica, you'll sometimes hear an accent that will make you think of English. It is true that the US has a lot of influence here and that can be felt even in the language. Logically then, a lot of Costa Ricans understand and speak English. So you should be able to communicate with people. I also recommend you watch some films in Spanish before you go, perhaps even with Spanish sub-titles so that you can get used to the sounds. If you are really motivated then you could read newspapers or a book in Spanish. Don't forget to take a pocket dictionary to help with simple vocabulary and try to learn some of the country's typical expressions! Some of them are surprising.
Other languages and dialects
There are a dozen other languages spoken in the country, several local languages such as Bribri with 11,000 speakers, Maleku or Guatsuo with 8,000, Cabécar with 9,000, Ngäbere and Guaymi with 5,000 speakers and Bocotá. On the Caribbean coast Mekatelyu, a sort of English creole that is very pleasant to hear, is spoken by more than 55,000 people. All these languages, which are a result of its history and culture, only add to Costa Rica's charm.