Keep calm and speak Bahasa Indonesia, The Republic of Indonesia is not only the largest archipelago in the world, it is also a large group of peoples and communities. There are more than 540 dialects but the official language of Indonesia is Bahasa, which is learned in schools and allows to facilitate communication between the different regions and islands of the archipelago.
Most Balinese also practice English since tourism is the most promising sector in the country. You may not be able to communicate in English with the Balinese in some more rural and outlying areas.
In Bali, the Balinese language also differs according to the area and the caste.
Learning Bahasa for a European may present some difficulties but the practicality of this language is that its alphabet has been romanized and therefore facilitates reading and access.
Moreover, unlike most other Asian languages, Bahasa is not a tonal language, which facilitates comprehension and expression when you will initiate for the first time.
Here are some daily notions that you may find useful:
Good morning, good afternoon, good evening
How are you?