On Windows at least (can't test MacOS at the moment), the default `lang` of web page when there is no attribute is actually the display language of my browser (in both my and OP's case, English), not my OS's (Japanese).
Furthermore, Firefox has a heuristic lang detection algorithm (!) based on content. I wrote an article about it 10 years ago, but not sure if it has been changed.
> the default `lang` of web page when there is no attribute is actually the display language of my browser (in both my and OP's case, English)
Uhh, let me check...
> MacOS Monterey, default system language is Japanese but obviously I browse a lot of English websites.
I don't think that OP ever stated that their browser is in English, just saying that they also browse English sites (and they're probably not using Firefox - its text rendering is super consistent across different systems). Safari (obviously) and Chrome (less obviously) in macOS defers to the OS for text rendering - hence the problem.
Furthermore, Firefox has a heuristic lang detection algorithm (!) based on content. I wrote an article about it 10 years ago, but not sure if it has been changed.