In our experience there were some people that knew English but the majority of people that we met did not. Even if you don't know that much, people really appreciate it if you give it a go. Expect to get wet - waterproofs are important. Have more than one change of shoes. If you are prone to car sickness make sure you have tablets for it When planning your individual project, make sure that it is not too involved or complicated.
Flexibility is the key.
You will be seeing and doing stuff that, as Master Card puts it, is priceless. Unlike the others on the trip, I was not a graduate student but the only undergraduate in the group. I found out about the class through the Undergraduate Catalog for the University as I was choosing the classes for my minor. I was hooked. How else could I get tropical field experience at such a bargain price - room, board, and transportation included.