How to enable Safari translation (beta feature) for a webpage?

Regarding the View->Translation->Translate to... feature that is in Safari running on Big Sur, how does Safari determine whether a page can be translated? A pointer to some documentation would be appreciated, if it exists. I've come across a few sites that Safari would offer to translate (button in address bar right) but I can't figure out what exactly needs to be done.

I've tried a couple things, setting <html lang="..."> , setting the Content-Language: ... via http or in a meta tag (which apparently is bad practice per Mozilla)

Couldn't find anything that explicitly addresses the feature in the Safari and WebKit documentation, looked at Intl web api but if that's it I'm at a loss at what to do (kind of doubt it would require the internationalization to be implemented by the page)
I found some examples for working/non-working websites, yandex Russian is detected and offers a translated version, but niconico Japanese isn't detected, I am not sure what differences are important.
How to enable Safari translation (beta feature) for a webpage?
 
 
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