Try watching Filipino TV sometimes. Some presenters will so for a stretch speaking Filipino or another language, then say some things in unaccented American English. I found it odd that some of the English words they used were not very esoteric, and probably existed in the Filipino language already (“fountain” seems to stand out in memory for whatever reason).
A phenomenon exists in Japanese where it is “cool” to use the English words. Bonus points if the English does not make sense in that particular context.
On a similar note, I find that English inserted into Indian songs always annoyed the piss out of me. In the last 10-15 years, Indians have been doing this, but now they have a familiar American accent when they speak English, and an Indian accent when they speak Hindi, and suddenly I like it! I discovered it was the Indian accent that grated on my ears. (And really, it is a silly accent.)
Teaching register is actually essential for high-school level work. It’s fine to use lots of slang in creative writing, not so fine in a business letter, and when writing an advert you choose which register to use depending on your audience.
My Phillipino step-mum’s Facebook wall often includes stuff like this:
Ooops, ambo masyado kasi a diprensya nin face mo now n then. yay hair cut bungat a namin change. main ka duman balak magpa-putoh one day?
palagay ko bagay ka nin short hair but not necessarily bobbed, siguro ha wedding day mo i-try mo yay layered shoulder length. suggestion bungat. hikoman, last time ko nag-short hair similar to ur photo was in 1988 ot lawo. kaKO NaGUSTUWAN A ITSURA KO never ana.
(She’s not a great typist - I had to add in some spaces there).
This is very common among people who speak English and another language fluently.
The woman on the other side of the wall from my desk at work regularly will go back and forth between Spanish and English on the phone. It’s much like SciFiSam’s stepmother, where it’ll just flow from one language to the other and back. It’s kind of fascinating to listen to even though I don’t understand Spanish.
But that’s the problem. By teaching that their one register is correct, they are by extension teaching that every other register is incorrect. It takes very little time to cover the concept that your own register is not wrong, just not appropriate in many situations. You can still teach proper English (or whatever language) without shaming the people who do not naturally speak that way.
Plus, if you haven’t learned anything about style by the time you are at least in high school, then your English teacher failed you.