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WCAG: 3.1.1: <html> elements with lang and xml:lang must have the same base language
The HTML document 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. The
xml:langattribute value, if included on the
htmlelement, must duplicate the value of the
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. Multiple languages are an issue for users who speak and access websites in multiple languages. It is essential to specify a default language and ensure that it is valid for screen readers to function correctly.
Screen readers use different sound libraries for each language, based on the pronunciation and characteristics of that language. Screen readers can switch between language libraries easily, but only if documents specify which language(s) should to read. If the language is not specified, screen readers read documents in the user's default language, resulting in garbled language. It can be challenging to understand anything when screen readers are using the wrong language library.