I am not sure what you mean with "invented". But an explaination would be that the grammars and syntax and all that language stuff lies within our brain. The rules and how a language appears is not random and invented from nothing, but influenced by many brains working thogheter.
Human languages is a window to how our brain is organized. We correct each other and learn to use language from each other all the time, this is culture and language evolving. That's why you find so many similarities among languages around the world, and also between sign language and spoken languages. Similar words and arts are also found in cultures that never have met.
