HTML / XML / JS, low-level validity (cross-cutting)MediumInferred
Mixed/invalid lang codes
What it is
Wrong language codes.
Why it matters
Confuses language handling/a11y.
How to fix it
Use valid codes consistently.
Impact
Low-medium. Inferred.
Evidence
Valid language declaration. Google Search Central, Tell Google about localized versions (hreflang); W3C, Web Content Accessibility Guidelines (WCAG) 2.2
Sources