It’s very common to use it on the HTML element, less common to use it on elements further down the tree. Like many accessibility features of the web, it’s often neglected by developers.

I live in a multilingual country and don't see it used as often as I would hope, even if it's more common here than elsewhere.

I often see it on elements/sections of sites that link to content in another language.

Here, German is the "main" language and lots of documents are available in German but not English. If you need to link to a German page/doc from an English one, it's not uncommon for the link text to be in German too.