What nonsense. The lang attribute in an individual html file inside an ebook DOES NOT override the overall language for the book. Your PocketBook developers dont seem to understand what an ebook is. The spec they quote is for a *single html document*, which is completely irrelevant for an ebook tht can be composed of multiple html documents each having their own potentially conflicting lang attributes.
The overall language of an ebook must be read from the metadata of the book. In the case fo EPUB books that means from the <metadata> section of the OPF file. Relevant spec:
http://idpf.org/epub/30/spec/epub30-...-metadata-elem