Does it specify the language of the metadata? Or something else?
The language of the book is already provided by dc:language. The pure xml has no language.
So perhaps if the package tag needs an xml:lang element it should match the first dc:language tag contents as the dc metadata (titles etc) should be in that language as well?
Does anyone know exact what that attribute on the package tag actually is indicating the language of?
Update:
All I can find in the epub3.2 spec is the following:
The xml:lang attribute on the package tag is *optional*.
In addition ...
Quote:
xml:lang
Specifies the language of the textual content and attribute values of the carrying element and its descendants, as defined in section 2.12 Language Identification of [xml]. The value of each xml:lang attribute MUST be a well-formed language tag [bcp47].
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So the only language dependent pieces of the package tag I can think of are metadata elements.
Unfortunately the lang of the metadata need not match (but typically does) the language used in the epub.
So my guess is we need to default this value to match the epub dc:language value.
Metadata elements can also use alt:language and xml:lang attributes to make things more complex.