The page does not say what language it is in

Why it matters

The lang attribute on <html> is how a screen reader chooses its voice and pronunciation: without it, English read with the visitor's default French voice is unintelligible. Browsers use it to offer translation and to hyphenate, and it is a Level A requirement of the accessibility guidelines. Google works the language out from the text and doesn't rely on it, so this is about people and browsers, not ranking.

Sources: WCAG 2.2, 3.1.1 Language of Page; HTML: the lang attribute, whose value must be a valid BCP 47 tag.

How to fix it

Put the language on the opening <html> tag, with a hyphen, not an underscore:

<html lang="en-GB">

en is enough if you don't want to name a region. In WordPress this follows Settings → General → Site Language.

Examples

These are the rule's own test pages, and what it says about each. They are run on every change to the rule book.

<html lang="english">
<title>T</title>

Warninglang="english" isn't a valid language tag, so it is ignored.

fail-english

<title>T</title>

WarningThe <html> tag has no lang attribute, so screen readers and browsers have to guess the language.

fail-none

<html lang="en_GB">
<title>T</title>

Warninglang="en_GB" isn't a valid language tag, so it is ignored.

fail-underscore

<html lang="en-GB">
<title>T</title>

PassedThe page says its language is en-GB.

pass-en-gb