[Quoth]

I think if the Metadata form entry for Language (Calibre Metadata) is blank the Editor removes the <dc:language>en</dc:language> line in content.opf if the default "Update metadata from Calibre" under Calibre integration is checked.

I think this is a bug. Surely the Language in the ebook should initially set the Calibre field and shouldn't be deleted if the box in the form is empty?

Also should it be en or English by default?

Edit: It seems like English in the Calibre Metadata form gives <dc:language>en</dc:language>
in the actual ebook?
Also is Editor -> Help -> Preferences per ebook or for any instance later opened of the Editor? I thought I unchecked "Update metadata from Calibre", but it's currently checked.