Speaking Spanish in Guatemala

I think it is indispensable to learn some basic of the language of the country you plan to live. I have learned Spanish in Antigua Guatemala from one of the numerous private Spanish school you find there. The teaching is on one to one basis, and if you are not English speaking native you will learn anyway from English speaking teachers.. so make sure your English is good. Now there are new destinations to learn Spanish in a more immersive environment than Antigua where you will tend to be mostly with English speaking people.
Also if you plan to live long term be able to speak some basics helps in your daily life.. but for any serious transaction like real estate or buying a car you might want to be accompanied by a fluent English - Spanish person so you don't miss important points.
Guatemalan in touristic areas mostly masticate some English so you can go by for your daily chores. When you are in the market bring a calculator so the vendors can write you the prices of the groceries if you don't speak enough.
I think being able to speak some basis is a sign of respect for the others.

Oh gosh YES! I tell this to everyone over and over -- learn as much Spanish as you can before you come here! And even then, be prepared to be stumped. 
It's helpful to carry a pen and paper with you for impromptu Pictionary. Draw what you're looking for, write down numbers, ask things to be written down. I have also done some amazing charades to find things I want, and made up crazy sentences using only the words I knew. ("At the school there is a black thing on the wall, I want to write on it. What is that?" Answer -- chalk, or tiza") 
Keep a positive attitude, say por favor and gracias a LOT, and remember that it took you years to learn your native language, so why should it be any different with a new one?