I will try to answer some questions. Now it's about the features of language: arbitrary.
The term arbitrary here refers to the fact that the symbol that people use is made or chosen without any principle, logic or reason. For example, people in Java do not have a principle or logic to call an animal with long tail, two hands, and two legs and like scratching and playing on trees as kethek. There is no logic either for people in Jakarta to call that animal monyet. This is what arbitrary means.
So, to answer Daniel's question: English people do not have any reason why they call the animal that produces "meauwww" as cat. Again, this is because language is arbitrary: no logical reason is used when choosing a symbol, and no direct relationship between the symbol and the symbolized.
Now that a group of people have chosen a symbol for something, all the members of the group should approve and use the same symbol to refer to the same thing. In the example above, the members of Javanese language society approve the use of symbol kethek to refer to that animal, while the members of Betawinese approve the use of monyet. Again, the symbol should be the result of the convention of all members of the (language) society. That's why language is called conventional.
To answer Wisnukurnia: How is language created?
When a group of people make a new symbol and the all the members approve, then they will make a new language, or at least they add a new word for their language. Remember, language is a system consisting of many elements that support each other.
Well, once the language is used by the people, it will exist. No society exists without language, and no language exists without the society.
Hope, it's clear enough now. For further info, please read Abdul Chaer. He has clear explanation about this.