If it's a HTML attribute, then you can only use it with DOM elements, with no control about when it runs. Instead, a JS API gives more flexibility and control.

If it's a HTML attribute, then you can only use it with DOM elements, with no control about when it runs. Instead, a JS API gives more flexibility and control. Besides, I think the "lang" attribute is supposed to signal what the language of the text inside that element is, not what it could/should be. So even if going with attributes would be the way forward, a new one would need to be created.