It would be good if you know how to speak Spanish, but the more educated people here in Panama speak English as well. Panamanian schools have English classes and the locals are proud that they are leaning English. Cashiers at the grocery stores and cab drivers only speak Spanish.
In many respects I feel bad, because when I first left the US, we went to Costa Rica, but I had a concierge then, who spent some time in the US, and whenever I needed help, I had him. When we went to Panama, I had another concierge named Hector. Looking back on it, if I had the chance to do it over again, I would not have had a concierge, because what you need to do is get in and try to learn the language. Having translators has kept me from learning Spanish and maybe I am too old to learn Spanish now, because it does not come easy.
My wife speaks Spanish so she is my translator now. Recently we had to make a phone call to customer care for our Hyundai and the service department did not speak English. I got my wife to talk to them on the phone and set up a service appointment for my car.
The other day, I was talking to a Canadian expat who owns the mailbox franchise here in Coronado, and he has been here almost as long as I have. He is unable to speak Spanish, but he has a business. They have four women working for them and one of these four women speaks English. So he communicates to the one who speaks English and she in turn translates to the others.
So overall, you would still be able to get by with just English. I know some people here who do not know how to speak Spanish, yet they are able to get by. It would of course be best if you learn even just a little bit of Spanish.