Whenever you create a site on Voog with more than one language it will automatically detect your visitor's language based on their browser settings or location.
In order to set a default language you'll need to go to Content > Structure and select the preferred language that you'd like to set as default from the header menu.
Which language visitors see? menu allows you to select the current language as a default for all guests. After you've done that, just click on the Save icon from the Voog user interface and the default language will be set.