The Tahitian language is the most spoken language of the indigenous Polynesian language so if you're in other parts of Tahiti you will hear people who live in Tahiti speak the Tahitian language.
Although most tourists areas and resorts in Tahiti have people speak the English language so you do not have to know the Tahitian language to get along in Tahiti.
However it can't hurt anything to know the Tahitian language but I believe there are some translators there in Tahiti although I've never been there yet.
I do hope to visit Tahiti though before I die and I've been working on speaking and learning the Tahitian Language.
Also about 8,000 people speak Marquesan in the Marquesas Islands.