Changed language field to adhere to RFC 5646

As mentioned in http://dublincore.org/documents/dcmi-terms/#terms-language, language fields should adhere to a controlled vocabulary such as [RFC 5646](http://tools.ietf.org/html/rfc5646). I've updated the schema to reflect that. I'm assuming documents in multiple languages should just be comma separated, though if I missed a place where the standard defines a better way to do that please edit accordingly.