Maybe the situation is due to lack of self-confidence.
Actually most Chinese Singaporeans are able to speak Mandarin
rather fluently.
Yes, it's true.
I know you may find this surprising.
It's just that sometimes people dont feel self-confident, or
they think they are not so fluent, so they switch to English words
and so on.
Actually I have an experience myself. Today I was calling
someone for a meeting, she told me she couldnt speak Mandarin, so
she asked me to talk to a colleague.
Actually after we were there (I am accompanying my relatives)
she can speak fluent Mandarin to them.
So, why did she say she couldnt speak Mandarin, and even plan to
ask for a colleague?
It's all due to lack of self-confidence, or over-estimation of
required Mandarin fluency.
Actually, if you "force" an average Chinese Singaporean to speak
Mandarin only, suddenly their minds are able to adapt and
adjust.
But the situation with Singapore is that, it is a
multilinguistic society, an average person knows several languages,
so, naturally, people sometimes code-switch (speak in mixed
languages) because the other party or the intended audience is also
a multilinguist person who understand this code-switching.
But if you force a Chinese Singaporean to speak to a
singular-language audience, for instance, to a visitor from China/
TW, then you will see his/her Mandarin fluency is not bad at
all.