Google's John Mueller confirmed that they completely ignore the HTML lang attribute for determining the language and region of the page. John said this at the 55:54 mark into the video hangout ...

But the language attribute within the HTML markup is something we don't use at all. We've found that this language markup is something that is almost always wrong. So we tend to ignore that." ...

In short, HTML keywords and tags control how a website looks and behaves. Without HTML, we'd be looking at plain text on blank pages. HTML has been around since 1989, and received a major update a ...