The official language is Tongan, but English is taught in the schools and is widely spoken in the main towns. Tongan is similar to Samoan. One major difference between them is the number of glottal stops (represented by an apostrophe in writing) in the Tongan tongue. These are short stops similar to the break between "Oh-oh" in English.
Every vowel is pronounced in the Latin fashion: ah, ay, ee, oh, and oo (as in kangaroo) instead of ay, ee, eye, oh, and you as in English. The consonants are sounded as they are in English.
An extensive knowledge of Tongan will not be necessary for English-speakers to get around and enjoy the kingdom, but you could learn a few words to elicit smiles from your hosts and to avoid the embarrassment of entering the wrong restroom.
The Friendly Islands Bookshop on Taufa'ahau Road carries language books, and the Tonga Visitors Bureau on Vuna Road distributes a brochure of Tongan phrases.