Panama is predominantly Spanish-speaking. Here in Bocas del Toro, as a tourist visiting for a week or two, you'll be able to get by if you only speak English.
But if you are planning on retiring here, do yourself a favour, and learn some of the basics of Spanish.
I studied audio lessons before I arrived, and continue to study now.
I have been able to employ local workers, and can negotiate and communicate with them in their own language.
I have learned much from them, and have enjoyed being able to work, communicate and laugh with them.
Surely one of the joys of going to live in another country is the thrill of learning to make yourself understood there. Just being able to successfully order two beers at a bar is a great first step.
Don't be the awful caricature gringo expat that refuses to learn a word of the local language, or any of the local customs or habits. Don't just hang out with the other expats who don't bother to learn a thing about their new home and neighbours. You will miss out on so much that your new homeland has to offer.
Make an effort to fit in, and become a part of your new home, not aloof from it, separated by a stubborn refusal to learn a few words of the language of the country in which you have chosen to live.