The human species is the only one that uses language to communicate thoughts and feelings. Other species can communicate by sign language yet there is no agreement among psychologists whether this is language as humans perceive it. The known categorized language components depend on our interpretations of sentence structure, semantics, and phonology.
Language development begins with prelinguistic vocalizations. We call this baby talk. The response from a caregiver to baby talk is conversation. A caregiver babbles back to a baby and somehow comprehension is present. This understanding precedes the first spoken word, represented by gestures and actions.
Communication begins at birth. It depends on the ability of two or more communicators to decode and express their intentions with shared meanings. Words are used as symbols and nuances, intonations, facial expressions and gestures are dispatched as communication too.
Syntax (the order of words) leads to increasing use of language complexity like pronouns, prepositions, and long conversations. Children continue to learn this by observation and imitation.
Children's Language Theories
B.F. Skinner theorizes that speakers (models) reinforcement of children's language skills facilitates their learning. When a child errors, the model corrects and reinforces the child so he repeats the language correctly. Models are necessary as social nurturing is essential for language development.
Chomsky theorizes that environmental influences plus an innate language acquisition tendency facilitates children's learning.
Piaget theorizes that children have cognitive development (understand symbols) before they can develop language.
So, language is known only to humans and it is developed as a system of oral communication.
Here is an example of how I built language skills (vocabulary) without limitations, with a child. When our youngest daughter was two years old, toddling, racing around our house, she was very curious about everything she couldn't reach. Meanwhile, I was very tired of using the word "no." It just wasn't useful for a precocious two year old. I tried "dangerous" instead. The event when this word was introduced was when she was digging, throwing, and tasting dirt from a potted plant that was bigger than she was. I didn't know if she comprehended it until the next day. A scissors was left on a table edge that she could reach if she was on her toes. She decided to investigate. I watched from a distance, she didn't know she was being observed. She stood on her toes and reached for the scissors. She touched them, stood flatfoot, turned and looked at me. She said "dangerous!" I loved it, she understood, and I stopped saying "no."
No matter how our communications through language develops, it depends on our interpretation of it to truly have meaning.