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.
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.
The 'Which language do visitors see?' selection allows you to select the current language as a default for all guests. After you've done that, click on the 'Save' button in the Voog user interface and the default language will be set.