Language of Page
The default human language of each web page can be programmatically determined.
In plain language
Every page needs to say, in its code, which language it's written in — so software (like screen readers) knows how to read it correctly.
Why it matters
Screen readers use the declared language to choose the right pronunciation and voice. Without it, text may be read with the wrong accent or pronunciation, making it hard or impossible to understand.
In practice
Fails this criterion
A page written entirely in Portuguese has no language declared in the code, so a screen reader defaults to English pronunciation, mangling every word.
Meets this criterion
The same page declares lang="pt" in its HTML, so the screen reader automatically switches to Portuguese pronunciation.
Test it yourself
Check the page's HTML code (view source, or browser dev tools) for a lang attribute on the <html> tag. If it's missing or wrong, this criterion is failing.