In plain language
Passages in another language must be marked with their own lang attribute.
Who it affects
Screen-reader users hearing mixed-language content with the wrong voice rules.
How to check
Find foreign-language phrases/sections and inspect them for a correct lang attribute. Worked example — A survey tool's question builder: inspect a lang attribute that appears to be copy-pasted or inherited incorrectly at the part level: verify it matches the actual language of that specific text.
Common misconception
This is easy to forget on multilingual marketing pages. A single foreign phrase or slogan dropped into an otherwise English page needs its own lang attribute, or it gets mispronounced. Real example: on a bank's mobile money-transfer screen, a Spanish-language help article is embedded inside the English transfer screen with no lang attribute marking the switch, so a screen reader keeps reading it with English pronunciation rules.