I plan to travel to Bali and want to learn some basic words in native language, like "Hello", "Thank you", ....
But I'm not sure which language is native to Bali. It looks like there are Indonesian (Malay) and multiple forms of Balinese language.
Which one should I choose?
The native language in Bali is bahasa Bali (Balinese). The official language in Indonesia is bahasa Indonesia (not Malay as you said, due to political reasons the two languages have separated). Anyway, Bahasa Indonesia is used by all people for education, government and almost everything else in Indonesia. You definitely should choose Bahasa Indonesia since it is spoken by all people in Bali and all other islands of Indonesia. Also, people in the touristic places will be able to understand basic English words and you can manage by English only, but it will be nicer if you use some Bahasa Indonesia words and people there will respect that.
Here are some few words that can help you get by:
Remember, the letter C in Indonesian is pronounced as CH.
P.S. I speak Bahasa Indonesia and I always managed by just Bahasa Indonesia anywhere in Indonesia.
Since Bali's main income is from tourism, you can survive by just using English, and the local people are able to speak English or at least "broken English" (This is not applicable to any other island in Indonesia).
If you want to use local language, you better choose to learn a bit of Indonesian words. Some basic words (other than what MeNoTalk has mentioned) can be found below :
