The average visitor to Bali will find English – at least a basic level – widely spoken. It's possible to go for days or even years without knowing much Bahasa Indonesian (the national language) or Balinese (the local language, still used among friends and family). However, as always, learning the basic words will be appreciated by locals.
Many visitors to Bali like to learn at least the basics of Bahasa Indonesia. South Bali and Ubud have many tutors who advertise in the same places you'll find rental listings. Otherwise, the best place for courses in Bahasa Indonesia is the Indonesia Australia Language Foundation.