Wouldn't be the first time InDesign was guilty of introducing problems.
Thanks for the input. I've purchased many epubs (both version 2 and version 3) over the years, but I've never purchased one from a commercial source that had more than one dc language entry in the opf's metadata section. This thread is the first place I've ever heard talk of it, in fact.
EDIT: the latest EPUB specifications state that more than one language element MAY be included for multi-language documents, but it doesn't explain what the advantages would be (nor how reading systems are expected to address multiple language elements). So as usual, everyone makes up their own rules as to how multiples are prioritized.