When you enable a language for the first time, translation files from localize.drupal.org are downloaded. Depending on the language, these may cover a large portion of strings in the Drupal admin UI, but there are likely to be some texts that are not translated. In addition, any text custom-created for the current site would need to be translated. The interface to translate these strings can be found by going to Configuration, then User interface translation, or by navigating to /admin/config/regional/translate.
On the User interface translation form, you can view all strings in the system, as well as locating ones that need translation into a particular language. You can select the appropriate language, select whether to see untranslated strings, translated strings, or all, and optionally provide text to be matched...
