> to my ear, I’ll say something perfectly coherent and pronounced exactly as the locals do
I noticed a similar thing listening to many English people trying to speak Spanish. I could hear that the native English speaker pronounced the vowel sounds of a Spanish word incorrectly - but that the English speaker could not tell. Very common if Spanish word learnt from reading and trying to pronounce it as English might. I also hear a similar reading mistake from other countries trying to speak English.
English can have extreme vowel variation - e.g. jokes based on bending vowel sounds to change word meaning. Spanish has a few vowel sounds and they seem very similar in different countries. English accents often change vowel sounds dramatically - so English speakers are not as aware of the importance of speaking vowels correctly. As a New Zealander, our vowel sounds trip up other English speakers.
I'm not sure how we learn to fix it when our hearing or sound formation is incorrect. Someone to incessantly correct one's mistakes does help but that level of patience is hard to find.
I know that I still can't hear or say nasal sounds correctly in other languages.
I think the issue here is that it's hard work for a native English speaker to keep track of the correctness of every single vowel sound because in English so many are elided or become "uh".
Listen carefully to different English accents, or even better try and mimic them.
There's a massive variety of vowel sounds in English: Sydney, Irish, Boston, Indian, etcetera.
English speakers can often hear the differences, and many people can produce the different vowels when mimicking the accents (country, city, person, foreigner).
I did not deny the fact that there is a greater variety of vowel sounds available in English. I merely doubt its explanatory power for the phenomenon you describe. But perhaps I am confused about exactly what that phenomenon is.
Actors and singers do it by hiring a voice coach - someone who doesn't just know the sounds, but can explain how to adjust your mouth muscles to make them correctly.
Most classes and individual teachers won't do that. They'll either think "Eh, good enough for a foreigner" and shrug, or they'll say "That's wrong" and repeat the correct sound at you, which won't fix the problem.
Sometimes changes happen in one language. There is a huge difference between the Received Pronunciation (RP) version of British English that was the standard up to around the early 90s, and the Estuary English that became mainstream after that.
I noticed a similar thing listening to many English people trying to speak Spanish. I could hear that the native English speaker pronounced the vowel sounds of a Spanish word incorrectly - but that the English speaker could not tell. Very common if Spanish word learnt from reading and trying to pronounce it as English might. I also hear a similar reading mistake from other countries trying to speak English.
English can have extreme vowel variation - e.g. jokes based on bending vowel sounds to change word meaning. Spanish has a few vowel sounds and they seem very similar in different countries. English accents often change vowel sounds dramatically - so English speakers are not as aware of the importance of speaking vowels correctly. As a New Zealander, our vowel sounds trip up other English speakers.
I'm not sure how we learn to fix it when our hearing or sound formation is incorrect. Someone to incessantly correct one's mistakes does help but that level of patience is hard to find.
I know that I still can't hear or say nasal sounds correctly in other languages.