I came across the video below a bit ago, and thought it was relevant to languages of different animals, but rather in children too young to formulate true sentences.
Click here to get to the link for the video (The video itself was formatted to prevent embedding)
In the video, it features two twin brothers having what appears to be a conversation, but without any true words. They are merely speaking to each other in sounds, and it brings the question, are they really talking, and can they truly understand what the other is saying? From the video, it does appear that there is some sort of established communication going on between the two, and they are able to converse with one another about things that others cannot comprehend. As cute as it is to see, it's strange to think that they have something important to tell one another, and we have no idea what they could possibly be saying. Although some can deem it twin telepathy, as they do seem to have the ability to sense what the other is thinking more than other siblings would, it appears to just be some form of language that the two made up and are able to understand. There are plenty of other videos in which the same can be seen by other twins.
Comparable to the songs of birds or the other sounds that animals make, the incoherent noises that the twins are making are examples of language, and how the brain allows for different individuals and different species to come up with their own efficient way of communication, whether it be the low frequency noises of the elephants or the screeching of one twin boy attempting to tell the other something about the refrigerator. It can ultimately be seen as some sort of language.