Belize is unique in that it is the only Central American country that has English as its official language.  All locals speak English fluently, in addition to either Spanish or the local Creole.  All signs, menus, labels, etc. are written in English, so English speaking visitors will have no problems communicating.  There really aren't any special local phrases used:  "Hello," "Please" and "Thank you" work just fine among locals and tourists alike.

Don't be surprised if you're out on an excursion and your guides begin speaking to each other in Creole.  They don't expect you to understand them, and they'll always use English with you without you having to request it.  Most are happy to tell you about their language and teach you a couple phrases if you are interested, but it is not at all expected.