Well, it is like the Biblical story about the Tower of Babylon - in the beginning, there was only one language and all the people in the world spoke it. However, because of many disputes and quarrels, they all separated and created numerous new languages. The same thing happened in the real world (if you don't believe in what the Bible says) - there used to be Proto-Indo-European language, but over time, it developed many new languages as people grew apart and created new nations.
The language spread from one Proto-Indo-European language to the more than 6,000 languages we speak today because there were different groups that migrated or moved to different places, since they had problems. Some of them moved because there was no more food or because of weather change. Those who would stay in one place and then they would farm developed a different type of language from those of their wandering groups. Over time the groups kept on spreading and expanding all over the country. Their languages changed because it depended where they lived or what they had and used. For example, if they were a family that traveled with animals they wouldn't use the same words as someone who worked on a farm, they wouldn't be plows and spades in their vocabulary. Later, developing their languages to their cultures helped them create and develop their cultures.
