Apple watch dictates in the wrong language

Hi everyone!

• What I have set up:

So I have an Apple Watch S7, I've been using dictation on it for a few months now, I just bought it for 2 months.

I have searched and follow instructions from the web.

I have Language and Region mirrored my iPhone, which is set to English, then Vietnamese, by this order

I have "Enable dictation" turned on on my Apple Watch and iPhone, "Dictation language" are ticked for both languages.

Both my iPhone and Watch are on the latest updates 16.3.1 and 9.3.1 respectively, at time of writing this.

• My problem:

Whenever I try to dictate in English, it works just fine. But when I swipe up from the bottom to choose Dictation in Vietnamese, it still tries to dictate Vietnamese in English, which sounds absurd. And by the way, Vietnamese dictation in Vietnamese still works on my iPhone.

And the weirdest part is: It used to work! I used to be able to dictate both in English and Vietnamese if I choose to, using the same method I mentioned above: swipe up from the bottom to choose dictation in Vietnamese. And just got this watch for 2 months, this problem couldn't have been around for longer than that.

Please advise! I've tried hard reset both devices, even unpairing the watch and re-pair, still persists.