In a recent hangout, John Mueller confirmed that Google ignores the HTML lang attribute to determine the page’s language. They do not use it to determine the region and language of page.
He also said that you should use hreflang markup. The hreflang attribute tells the language of the document in the link. For Example.
<a href="http://www.yourdomain.com" hreflang="en">W3Schools</a>
Here is the question.
So how important is it to display in the HTML code, the language of the site, if you already declared it in Google Search Console, for example.
Here is the response from John Mueller.
We don’t use that at all.
So, we use the hreflang links if you have that if you have different language versions. 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.
You can check the response in the video below.
You can find the Google + discussion here.