CubeCart will use the 'master' definitions.xml file if there isn't an overriding key phrase in any of the actual assigned language files.
When assigning, there is the default language that is chosen in Store Settings. But that default can be overridden by choosing which language the logged-in admin wants to personally use, and also which language was being used when a customer registered.
An admin can choose which language to personally use by editing their admin profile in the administration's Administrators page.
When CubeCart is initially being setup, the language the admin acquires is chosen at step one where there is a drop-down selector that says "Update" (the default language).