Let's say that You want to add Spanish language to Your site.
First of all You need to download end enable following modules:
drush dl drush_language
drush dl l10n_update && drush en -y $_
And now add Spanish language and download translations of drupal core and contributed modules:
drush language-add es && drush language-enable $_
Ofcourse, instead of do this, You can manualy download and unpack .po file and import it using Drupal UI... Everyone has a choice :)