Not setting xml:lang in the package document is considered a violation of SC 3.1.1 Language of Page as without the language the metadata may not be read properly.
It doesn't appear that Ace is currently checking that a global language is set, however, but it would be helpful if a missing language declaration were reported.
danielweck
commented
2 years ago
Not setting xml:lang in the package document is considered a violation of SC 3.1.1 Language of Page as without the language the metadata may not be read properly.
We have a technique for this in the techniques document: https://www.w3.org/TR/epub-a11y-tech-11/#lang-001
It doesn't appear that Ace is currently checking that a global language is set, however, but it would be helpful if a missing language declaration were reported.
/cc @GeorgeKerscher