How do I install Drupal with a custom language?
Seems this works fine:

$ drush si standard \
    --db-url='mysql://[db_user]:[db_pass]@localhost/[db_name]' \
    --account-name=joe --account-pass=mom \
    --site-name=Example \
    --site-mail=noreply@example.com \
    --locale=it