It may seem obvious to say that each medium is constructed differently, noting for example the difference in constructing the messages in a book compared to constructing the messages in a film. At its most basic level, a book requires the creator to write or type thoughts onto paper or a computer screen, while a film requires many creators to work together in creating a product that includes sound and live or animated motion.
Consider the language used in photography to describe the process of creation. Photographers use terms such as "wide shot", "close up", "deep focus", and "f-stops". Someone writing a book would probably be more concerned with "chapters", "logical flow", "character development", and "story". Each medium uses its own language or process of creation, and this process makes each medium unique.
This isn't to say that language can't be shared among various media. Convergence brings together two or more mediums to create a new one. This may seem like a new term, but consider movies, a convergence medium created through the combination of theatre and photography. In this case, movies contain language from both theatre ("staging", "actor") and photography ("close up", "film").
To some, the concept of a unique language may not seem apparent, especially when analyzing and comparing messages from the same type of media. Consider the medium of television - all messages constructed for television must be created using cameras, microphones, writers, directors, actors, etc. But television is so broad, that messages in different types of content on the same medium can be constructed using their own language.
As an example, compare the construction of a television commercial and a reality show. Both are created using the same language, but they are created for different purposes and so must be constructed differently. Another example of this is a fiction book versus a non-fiction book - both are books, created using a set of rules unique to their medium, but both hope to achieve different goals (to entertain or to inform), and both have a unique language within their specific area.
It's important to understand that each medium has different characteristics, strengths, and a unique language of construction.