Would it be okay as an Indonesian of Chinese descent to speak Indonesian only in Singapore and refuse to speak English or Mandarin?

Answer 1

This is an interesting question because it surrounds the concepts of culture and language. You can choose to speak what language you want.

Answer 2

Singaporeans that I personally know are respectful about people's language preference.

However since Indonesian is not an official language of Singapore and the percentage of Indonesian speakers are limited, you might find it difficult to expand your social circle if you only speak in Indonesian.

On the other side, if you do speak English or Mandarin, using them as a way to connect with potential customers or locals would be a profound cause for your success