<html> element must have a valid value for the lang attribute
Moderatehtml-lang-valid
What it checks
Ensure the lang attribute of the <html> element has a valid value
How to fix it
Review and fix this accessibility issue following WCAG guidelines
WCAG
Success criteria 3.1.1. See the WCAG 2.2 quick reference.
More detail: Deque University's page on this rule.