Fastest way to add a new language to Drupal site.

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 $_
drush l10n-update-refresh
drush l10n-update

Ofcourse, instead of do this, You can manualy download and unpack .po file and import it using Drupal UI... Everyone has a choice :)