I do that when I don’t know the Chinese word for what I’m saying.
It’s called Singlish for the most advanced version. 
Personally I don’t think very high of the level of english speaking in Taiwan. Sure I have met a few people in Taiwan, their english is awesome. Mostly people who are interested in living in the USA or Canada. Or who studied abroad, are like that.
I just have met a ton of people who have qualifications from local universities, went to cram schools to learn english, have certificates to prove their english have top grade certificates. And when I try to talk to them their english is so bad it gives me a headache. Here those diplomas and certficiates are given on a pay your bills and show up and you get the paper with a high grade, I believe.
If you make plans to move back; it should not be solely on the basis of english speaking ability, but for their education and social development as a whole, imho.
It’s not really a problem.
As for choosing which language, they pretty much deal like anyone who’s multi-lingual.
Kids who grow up with OPOL being conscientiously applied don’t even grok that there are two languages until they get bigger.
Before they’re 5 or 6, it’s just like Mamaspeak and Dadspeak.
