As you add new modules, the available translations will grow. The Interface translation module provides a translation status report that is accessible from the Reports page. This will check the default translation server for the project and check whether there is a .po available or if it has changed. In the event of a custom module, you can provide a custom translation server, which is covered in Providing translations for a custom module.

If an update is available, you will be alerted. You can then import the translation file updates automatically or download and manually import them.