I hear the term design language a lot, not in the sense of the vocabulary. For example the Apple Computers built in the 80's had a Frog Design Language for the characteristically 80's look for their hardware. Microsoft recently introduced the Metro design language mostly for the software interface. Are there any resources out there (books especially) that could teach you to create a design language for a range of product's?
Frog Design is an industrial design firm that did a lot of early design for Apple hardware. In that context, "Frog Design Language" would refer to the overall style that was recognizably Frog's.
"Metro Design Language" is a fancy way to say "Metro UI Style Guide". Well designed systems--be they Corporate Identities, User Interfaces, etc, will have a defined set of rules and guidelines often wrapped up in something called a 'style guide' or, in this case 'design language'.
Bottom line is that the term isn't terribly specific and appears to be a term of art used in various ways.