When enabled, the Locale core module allows you to present your Drupal site in a language other than the default (English). You can use it to set up a multilingual web site or to replace the elements of the interface text with text which has been customized for your site. Whenever the Locale module encounters text, it tries to translate it into the currently selected language. If a translation is not available, the string is remembered, so you can look up untranslated strings easily.
For Drupal 6 and 7, you can install the Localization Update module to automatically retrieve translations from http://localize.drupal.org. For languages where translation has already been done, you do not have to do anything other than set up this module. This module will be included in Drupal 8 core.Read more