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Its in Dublin Core, to which calibre adheres for most things - if you wrangle 'und' into an epub content.opf, then at least one epub viewer/reader (can't recall which one) renders it as gobbledegook. Which is supportive of KG's position on this.
I regularly download PD epubs from a source that intermittently puts 'und' in the content.opf of some of their epubs.
And I'm not sure one can have a "list of languages for a book", the DC standard defines the Language element as "A language..." - singular, it recommends RFC4646 be used.
BR
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