We can see the language policy in hk education system.

When we put it into our case, as Mr and Mrs Lee want their 2 daughters to be a local student in HK, their  top language priority must be Chinese and English. Even they have the capacity to take up other languages, our education system only offers 6 language subjects in the public exam, the two daughters may therefore lack the incentive to learn the proper and formal usage of Thai academically.

Even now, when they want to pick up Thai, they might think it is too late to learn to be a native speaker.