This isn't what actually happens in practice though. In practice, children speak the language their school tells them to speak. And generation by generation less and less people speak the other language, and in smaller and smaller areas.
I'm somewhat familiar with the situation in Taiwan. No one is actively suppressing Hokkien (or Taiwanese or whatever you want to call it) anymore, but the language of instruction is still Mandarin and so Hokkien is dying rapidly. Even in cases where both parents speak it, often their children can just understand it and don't speak it, and there's zero chance their children - grandchildren of hokkien speakers - will.
Used to work with some Chinese Malaysians from Penang. All under 30, all spoke Mandarin to each other because the level of Hokkien knowledge ranged from "fluent" to "none at all".
For practical reasons alone most people are going to focus on those top 3. I know some people who are reasonably fluent in no less than 5 languages but it's not that common and you will obviously see at least two or 3 are passable at best. These are going to happen even if the schools could somehow teach 4-5 dialects + standard mandarin +_ english + malay!
Malay: National Language
English: International langauge
Hokkien: Mother tongue
Mandarin: ???
I genuinely don't understand why a brand new language that wasn't traditionally spoken into the area was introduced.
And in those days of the early 20th century when the foundations of formal education were laid if one was an educated person it would have been either in Mandarin or English (that said HK Cantonese is/was a huge pop culture influence but I don't think it ever was in an pan-sino educationist sense)
Yeah, the strong desire to do that is something I don't understand. But thanks for getting to the crux of it - it's all about having a chinese exclusive lingua franca.
A unified Chinese identity takes precedence over the local one, this is even more so when you are at the mercy of divide and conquer techniques. The only other alternative was Cantonese but then why would the Hokkien buy into that and vice versa? Educational resources are thin enough as it is.
English Mandarin Malay Then one of the various major dialects - and even then once you start moving around the country the Penang hokkien speaker in KL has to either switch to english or mandarin and the KL cantonese speaker in penang has to switch to mandarin and english and at the end of the day, with both the PRC and ROC using Mandarin as the common tongue, and the same situation existing for any other sinophere country bar Hong Kong...there we go.
If you go overseas and meet a random other Chinese person, your heuristics of common language are again going to skew
English or Mandarin then Cantonese then only one of the others so...
"they punished us at school if we spoke $MOTHER_TONGUE instead of $NATIONAL_LANGUAGE".
Care to explain what part of the comment you take issue with?