Drupal allows you to translate the interface to a language other than English. This page provides an overview of the installed languages. You can add more languages on the add language page, or directly by importing a translation. If there are multiple languages enabled, registered users will be able to set their preference. The site default will be used for users without their own settings, including anonymous visitors.
There are different approaches to translate the Drupal interface: either by importing an existing translation, by translating everything yourself, or by using a combination of these.