There appears to be an issue with Indesign generating a language tag in an ePub's metadata not according to the content but according apparently to the language version of Indesign or the primary system language used to create it (I'm not entirely sure which). I was producing an ePub in Chinese in CC 2015 Mac, and there were several languages being exported in the metadata. So I went through and checked that all text was marked as Chinese but the ePub was still being exported as Japanese (I double-checked that there was no Japanese content, even on master pages, hidden layers etc) . Then I remembered that the document had been created by a Japanese person using Japanese Indesign. So I did some tests, like creating a document with only Chinese content in Chinese Indesign with Chinese as the primary system language, and indeed then the only language in the ePub metadata is Chinese. Or creating a new document in English Indesign/English system but with only French content, and again the ePub was exporting with both English and French metadata. So it seems there is some sort of document default language being stored when the document is created which is determined by the environment and then exported in the metadata. Surely this is a problem for people creating ePubs in languages different from their system or Indesign language? Wouldn't it be simpler to allow the user to specify the language in the export dialog?
Have you checked the Language being specified in all used (and unused) paragraph and character styles?