Looks like an interesting topic here. This comes from someone who is hopeless at language skills. I only speak English, and picked up Hokkien in NS.
Firstly, Malay is the National Language of Singapore, there is no dispute about this - the National Anthem is in Malay. Singapore has mixed ethnicities in her social make up, and so different languages are heard. Because of commerce, English is the most widely used, but most residents' mother tongue is of some other origin, so a creolized language called Singlish has evolved. Singlish has components from commonly used words and phrases spoken by residents of the island. Even English radio DJs deliberately resort to Singlish to connect with the local audience. Until some future generation decides to replace Malay with Singlish for the National Anthem, Malay is the National Language of Singapore. And for the yellow-skinned Chinese, your mother tongue is probably Hokkien, Teochew, Cantonese, Hainanese, Hakka or some tribal derivative, it is NOT Mandarin.
Cheers!