Because of XWIKI-18477 the page translation REST resource returns empty default locale for any page translation that was created by the user. In order to determine the default locale you need to make another HTTP request for the original document which is a pain and a waist of resources and time.
For instance, create the Romanian translation for Sandbox.TestPage1 and then access:
The default translation is empty. You need to make another call to:
in order to determine the actual default locale.