In a Google Webmaster Hangout, Google’s John Mueller said that Google does not use the HTML lang attribute to determine the language and region of a web page.
The HTML lang attribute looks like that: <html lang=”fr”>
Google does use the hreflang attribute to find different language versions of a page. However, they ignore the HTML lang attribute because it is often wrong:
“We don’t use [the HTML lang attribute] at all.
We use the hreflang links […] if you have different language versions [of the same page]. 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 view the full video here: