If you're traveling on your own at least one person in your party should be able to speak a little bit of Spanish. Spend some time before your trip memorizing a few key phrases from a travel book or language tape. Know how to order food, buy items, ask for directions, etc. The people in Central America are generally very friendly, and they appreciate it when you try to speak to them in their own language. Even though you may think you sound foolish or may be grammatically incorrect, you can still get the basic idea across, and they appreciate that you made the effort.
You should be prepared for the possibility that you may need to converse with someone about an important topic, and there will be no one around who can translate for you. An example for me was when my friend and I drove our rental car across the border from Guatemala into Honduras. We had to go through three layers of Customs (Immigration, Police, and the office that handles foreign vehicle admittance) on each side of the border, and no one in any of the six offices spoke any English. Between the two of us we were able to get our point across regarding purpose of our visit, length of our stay, when we would return, etc. I think it helped that we were patient, smiled a lot, tried to speak their language (I think it's rude to expect them to speak yours), and acted grateful for their help. It was also very important to have the right paperwork. We had obtained written (and notarized) permission from the rental car agency in Guatemala City to take the car out of the country. This paperwork was essential at the border (along with our passports, of course). One office on the Guatemalan side of the border took our permission letter and hung it on the wall, awaiting our return in a few days. However, after we crossed the border (100 feet away from the Guatemalan Customs building), we went through the Honduran Customs offices, where an armed officer asked us for our permission letter for the car. Despite our best attempts to explain in Spanish that the letter was being held in the Guatemalan office, he still insisted that we provide the needed document for the car before he would give us the required stamp so that we could proceed. In desperation I got the car registration out of the glove compartment and showed that to him. It did have some official seals and other markings of importance on it, and must have looked okay to him, because he took the registration, looked it over intently while saying "hmmmm....um-hmmm.....hmmmmm" as he ran his finger down the page. Evidently, though, he couldn't read, because he was holding the page upside down! But it looked like an official document, and he went through the motions to fulfill his duty, and we got the stamp we needed. It just goes to show that you need to be both prepared AND flexible.