The main languish spoken in most areas of Tahiti are French and Tahitian.
So it can be a good idea to try and learn some of those languages if possible before going to Tahiti but it's not necessary to learn those languages.
The good news is that English is well spoken in the tourist areas of Tahiti which is where you'll be spending most if not all of your time at in Tahiti.
The people in the resorts of Tahiti also speak English pretty good.