They should start with what is their definition of language. To me it's any mean you can use to communicate some information to someone else and they generally get a correct inference of what kind of representations and responses are expected is the definition of a language. Whether it's uttered words, a series of gestures, subtle pheromones or a slap in your face, that's all languages.
For the same reason I find extremely odd that the hypothesis that animals don't have any form of language is even considered as a serious claim in introduction.
Anyone can prove anything and its contrary about language if the term is given whatever meaning is needed for premises to match with the conclusion.