Originally Posted by HarryT
It means exactly what it says: that the "dc:language" field in the OPF file (a mandatory metadata item) isn't there. Edit the OPF file in the ePub and add it. Its value should be a standard ISO language code, such as "en-GB" for British English, or "de-DE" for Germany.
thanks for answering so fast. Sorry that the question was maybe not very clear, but I ment: How can I edit Metadatas of an epub (in OSX), - but I have already found something for free.