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WCAG: 3.1.1: <html> element must have a lang attribute
The HTML document element must contain a valid
langattribute or must correspond to a valid
langcode for multilingual screen reader users who may prefer a language other than the default.
When configuring a screen reader, users select a default language. If the language of a webpage is not specified, the screen reader assumes the default language set by the user. Language settings become an issue for users who speak multiple languages and access website in more than one language. It is essential to specify a language and ensure that it is valid so website text is pronounced correctly.
Screen readers use different sound libraries for each language, based on the pronunciation and characteristics of that language. Screen readers can switch between these language libraries easily, but only if the documents specify which language(s) to read and when. If the language is not specified, the screen reader reads the document in the user's default language, resulting in a strange accent! It is impossible to understand anything when screen readers are using the wrong language library.