What is measured
The audit reads the root html lang value and records when it is missing.
Structure and on-page signals
HTML document language
This check finds the lang attribute used by browsers, assistive technology and text processors.
Pass criterion
Russian pages declare ru, English pages declare en, and the language switch links equivalent versions.
What to fix
Set lang in the root layout and derive it from the actual route locale.
Automation limit
lang does not replace hreflang or describe the relationship between translations; those signals solve different problems.
Reference