In plain language
The page's language must be declared in code (lang attribute) and be correct.
Who it affects
Screen-reader users who need correct pronunciation, and tools that depend on language metadata.
How to check
View source: <html lang='..'> must exist and match the page language — including after switching site language. Worked example — A hospital's patient portal appointment scheduler: switch the site's UI language and check document.documentElement.lang in DevTools console: if it doesn't update to match, it fails.
Common misconception
Setting lang="en" once in the template feels like a one-time task. It breaks the moment a site adds a language switcher and forgets to update the attribute when the user changes languages. Real example: on a music streaming app's playlist screen, the playlist screen's HTML omits a lang attribute entirely, so assistive technology has no way to know the page's content is in English and can't pick the correct pronunciation rules.