Previously, it was believed that the HTML language attribute matters to Google and is a way to determine the language and region.
However, recently, contrary to this popular belief, Google’s John Mueller said that they completely ignore the HTML lang attribute. According to him, you should rather use the ‘hreflang’ markup.
Someone asked, “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.”
To this question, John replied,
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.
So, it is now clear what you should be using to let Google know your language and region. Work accordingly, webmasters.