The default language is the language that applies first when a visitor navigates to the website. Normally you specify the language that you believe the majority of your target group wish to read as the default language. If the visitor wants another language, an active action is required from them. Read more about this in "Which Language is Shown to Visitors?".
The default language for the entire website can be changed in web.config. You can do this by changing the language encoding in the uiCulture attribute. If uiCulture is not defined in web.config, the website will use the first enabled language in the list of website languages in Admin mode as default.
<!-- GLOBALIZATION
This section sets the globalization settings of the application.
-->
<globalization culture="en-US" uiCulture="en" requestEncoding="utf-8" responseEncoding="utf-8" resourceProviderFactoryType="EPiServer.Resources.XmlResourceProviderFactory, EPiServer" />