I got stuck on this "missing required element "dc:language"" error too. I am posting my solution for anyone who may find this thread. I create epubs from InDesign on a Mac and got this error when uploading through iTunes Producer.
Here's what you do on a Mac:
1) Download Springy from springyarchiver.com. Open Springy, and go to File > Open.
2) Locate your epub file and click the Open button.
3) You will see a small file menu. Go to the OEBPS folder and you'll see the content.opf file. Double click it to open with TextEdit.
4) Add the code <dc:language>English</dc:language> within the list at the top. I don't think it matters where (but maybe it does). Close the file without saving.
5) Springy will then ask if you wish "Overwrite the item in archive." Click Yes.
6) Close Springy.
Congrats, you've just edited your metadata! Your epub should be all set for upload to iTunes Producer.