HTML elements with lang and xml:lang must have the same base language
Moderatehtml-xml-lang-mismatch
What it checks
Ensure that HTML elements with both valid lang and xml:lang attributes agree on the base language of the page
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.