Google Assistant's language support between different platforms is a confusing mess that I won't pretend to be able to understand, but the good news is that over the past year, more languages and variants have been added. We've seen Spanish and Italian recently, but today's addition is different: Assistant on phones should now work in Singapore English.
So if you live in Singapore and have set your phone to Singapore English as a language, you should start seeing Assistant soon (already?) when you tap and hold the home button on your phones, instead of the Google Now on Tap / Screen Search that was before it. This should work on all Android phones with 6.0 and above.
Singapore English joins 5 other English variants in being supported by Assistant: US, UK, Australian, Canadian, and Indian. However, unlike some of these, it isn't yet available through Assistant on other devices like TV and Home.