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WCAG: 3.1.2: lang attribute must have a valid value
The language specified in the HTML document must be one of the valid languages to ensure text is pronounced correctly for screen reader users.
When configuring a screen reader, users select a default language. If the language of a webpage is not specified, the screen reader assumes it is the default language set by the user. Language selection becomes an issue for users who speak multiple languages and access the 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. If the language is not specified, the screen reader reads the document in the user's default language, resulting in a confusing experience!