Ferdinand de Saussure, a Swiss linguist, said that languages have two aspects. He called them "la langue" and "la parole".
"La langue" is the static structure of languages. From the synchronic perspective, languages are a bunch of rules. Every language has its own grammar and every word has its own meaning. We talk with each other following the rules of our common language, so we can understand what we mean. If languages have no rules, the world would be "the Tower of Babel".
But who decides the rules of languages and where are these rules? Laws are established by congresses (or kings), and we can read the law in the national archives.
When we are born, "la langue" already exists. And then we learn the rules of the language. In this sense, the rules of languages are "givens", which we can't change by ourselves. Especially we, the learners of secondary languages, are learning the grammar and vocabulary through textbooks and dictionaries, as if they were the inflexible written rules.
But we can't point out who decides the rules of languages. "La langue" is established by "la parole", which is the assembly of speeches. The rules of languages are just conventional. The meaning of the certain word established by the fact that people use the word as the certain meaning. For example, in English the word "dog" means the certain species of animal, because English speakers call this animal "dog". In Japanese "犬(inu)" means this animal, because Japanese speakers call it "犬(inu)".
Linguists study the convention of languages and write textbooks and dictionaries. But textbooks and dictionaries aren't the rules of languages themselves. They are just imperfect descriptions of the rules of languages.
In the short term "le langue" defines "la parole" and in the long run "la parole" changes "le langue". The relationship between them is dialectic. We experience that "le langue" defines our "parole", but it's difficult for us to recognize that "le parole" changes "le langue", because this process is so slow and in large scale.
Now I'm really interested in the family langue of my friend, Nigist Aisha. At first please read her journal on lang8.
"My Family's Language" by Nigist Aisha
"Le Langue" of her family language is establishing by "la parole" just now. She can observe "le parole" is changing "le langue"! It's a linguistic laboratory.