Use lang attribute in HTML output
The text in the interface (table headings) should be “en” by default, but the string should be translatable. This way, the german translation can change it to “de” and so on.
The Text from the book should be marked as the language of the book or page.
The language of a book should be stored in the OPF file and usually is. The language of the page can be stored in the
<html> tag and often is.
The language of the paragraph, sentence or word can be stored in any HTML tag. We should be able to keep track of language changes similar to the way we keep track of page changes. It's going to be a bit tricky if the language changes inside the match.
Plan: Use the book-wide language for now and implement language change detection later.
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