Search and delete any erroneous language declarations
Move language declaration up to the root HTML line
Adjust the <title> element
Run it through EPUB-Checker and Ace by DAISY
I would encourage all ebook developers to think of an InDesign-exported EPUB as a starting point. It is not a finished product, not if you care about accessibility or good development best practices. This list is a set of post-export cleanup instructions that me and the ebook developers I work with use to cleanup an EPUB.