`html.lang`

Default: error
Plugin: html

The <html> element has a valid lang attribute.

Why: Screen readers use lang to read the page aloud with the right voice, and browsers use it to translate and hyphenate. Google does not use it: it reads the language from the visible text. The value is a language tag, such as en or pt-BR, not a locale name, such as en_US.

When off: pages without a language, or with an invalid one, pass.

<!-- ✗ Fails -->
<html></html>
<html lang="en_US"></html>

<!-- ✓ Passes -->
<html lang="en"></html>
<html lang="pt-BR"></html>

References

WCAG 2.2: Understanding Language of Page: Pages must declare their language (Level A).
axe: html-has-lang: The same check in axe-core.
axe: html-lang-valid: axe-core also checks that the value is valid.
RFC 5646: Tags for Identifying Languages: The format of a language tag.
Google: Multi-regional and multilingual sites: Google reads the language from the visible text, not from lang.