…WHATWG HTML bug (26942) where someone asked why do these examples of lack the lang attribute? I thought the answer from Hixie was a bit dismissive and not based on any data or real-world benefits of use, particularly in the context of screen readers: Why not? Realistically, few people include it. It just means the language is unknown.
At the time, I could not get the latest archive to download from WebDevData.org (though that has changed, see below), so I fell back to asking for help on why the lang attribute is valuable.
How the lang Attribute on Is Used