In the User interface translation form, there is an Export tab. This form will provide a Gettext Portable Object (.po) file. You can export all the available source text that is discovered in your current Drupal site without translations. This will provide a base .po for translators to work on.

Additionally, you can download a specific language. Specific language downloads can include uncustomized translations, customized translations, and missing translations. Downloading customized translations can be used to help make contributions to the multilingual and internationalization efforts of the Drupal community.