Set the HTML lang attribute to describe the page’s actual predominant language. Then verify that the delivered page and its language label agree. This helps users and assistive technology; it should not be presented as a command that makes Google detect a language or target a country.

Google explicitly says it uses algorithms, rather than HTML lang or hreflang, to detect a page’s language. That makes the technical diagnosis more precise: a wrong language label is worth fixing, but an unwanted search-language result needs its own investigation. Do not promise that changing the attribute will resolve both.

Keep three observations separate

Take a fictional documentation page intended for French-speaking readers. Its route contains a French-looking path, most of its explanatory text is French, but a shared layout still declares English on the root HTML element.…