I say this, because in matters of learning, the pedagogy, the mechanisms, especially when it comes to the learning of language and written language, is HUGE. Some of it is burned in (Chomsky) and some of it not. Some of it relates to age. Some of it relates to sound vs shapes. Its not simple to say "because I did this, it works for a vietnamese person" because the scripts and intonations and a whole bunch of stuff are different.
How well does it work to teach Chinese to people assuming left-to-right scanning when chinese ideograms can be read up to down? Or how about when its boudustrophon? How many written works assume left to right, and actually make little sense for a right-to-left reader?
In many economies, and I include my own (Queensland, Australia) the whole word vs phonics debate became hugely political. Its riven the early education community: It is now actually quite hard to have a rational conversation about teaching and learning in early childhood now.
Maths ability is similar. Start talking about rote learning and the "times table" and watch people foam at the mouth.
Kids have amazing models of abstraction in part wired in. This is why "I shutted the door" happens: its not because grammatical errors are in the child: its because rational abstractions Shut -> Shutted happen in the model