What do you mean by "design language"? A programming language, a programming language used for design (like HTML, CSS), a the style of language used in their websites (e.g. Lorem Ipsum), or something else?
From wikipedia: A design language or design vocabulary is an overarching scheme or style that guides the design of a complement of products.
In microsofts case it's the "metro" style, with lots of bright (almost cartoony) colors, big blocks filled with lots of light-weight, sans-serif fonts.
