Language of Parts
The human language of each passage or phrase in the content can be programmatically determined, except for proper names, technical terms, words of indeterminate language, and words that have become part of the vernacular of the immediately surrounding text.
In plain language
If part of the page's text is in a different language than the rest (like a French quote inside an English article), that specific part needs to be marked in the code as being in that other language.
Why it matters
Screen readers use language markers to switch pronunciation rules. Without marking a foreign phrase, the screen reader tries to pronounce it using the page's main language rules, which can turn it into confusing, mispronounced gibberish.
In practice
Fails this criterion
A German phrase appears inside an English article with no language marking — an English screen reader mispronounces every word using English pronunciation rules.
Meets this criterion
The same phrase is wrapped in code that marks it as German, so the screen reader automatically switches to German pronunciation for just that part.
Test it yourself
Look for any foreign-language words, quotes, or phrases on the site (not counting proper names or common loanwords). Check the code to see whether they're marked with a different language than the rest of the page.