Even though everyone told us we'd struggle without learning Spanish before we went, we found the basic guide book Spanish covered most situations eg markets and haggling, asking for directions, and manners and pleasantries. We did speak to people who were backpacking it around and given the nature of the way they were travelling, having to speak to more locals, get transportation, stay in hostels, etc, they definitely had to speak Spanish more.
English was often spoken in the tourist areas, especially along coastal destinations. Even people with broken English, wanted to practice their English, even if we spoke in Spanish to them.
The people who spoke the least English were often police officers or guards. This worked to our advantage, as we'd often play the foreign 'don't understand' card and they'd give up trying to get bribes out of us.
Mexico was the hardest country to travel in, as they had the most people who couldn't speak a word of English, but it was also our first country to visit, so we hadn't picked enough of it up at this point. Playing charades and adding 'O's' on the end of words helped a little. The common guidebook phrases and Spanish mechanics massively helped though.
Besides Belize (predominately English speaking country), Spanish is the official first language of the other Central America countries. Costa Rica also has a heavily English speaking culture though. We didn't come across anyone who didn't at least have a basic grasp of English there.
Creole is also spoken along coastal areas, and there's indigenous languages spoken, especially in Guatemala, but they tend to also understand Spanish.
The language barrier wasn't as hard as we thought, but in hindsight, we would've liked to have done lessons somewhere on our route, so we would've been better prepared for South America and interacted with the locals a lot more. This probably would've been in Guatemala, as it's so cheap and common there.
By the end of 4 months in Central America, we had enough Spanish to get by and understood a lot of what was being said, but we definitely could've been better than we were.
As far as advice goes, at least learn the guide book Spanish before coming out and if possible, consider taking lessons either before, or during your trip in Central America. It can only be a good thing.