My standard of mandarin is very poor (GCE O attained F9, without tuition!). At the food courts and to some locals, I speak Hokkien, sometimes a little malay (sikit-sikit). When faced with a situation where the other person speaks only mandarin, I back off, go to another stall, there is always a choice. I've gotten by my entire life with mostly english and a little hokkien, malay. I don't think the term mother-tongue should refer to mandarin, but it is a losing battle. Most Singaporeans of chinese ancestry are comfortable with mandarin and even some Indians, Malays and Eurasians are getting good at it too. Its up to you guys.
Cheers!