Where did drush language-add disappear?
The problem is caused by not installing drush_language in the correct directory.
If you stil have the problem:

First remove anything related to drush_language:


Go to your drupal folder and do drush pmu drush_language
Go to /drupal/sites/all/modules and delete the directory drush_language, if it's there.
check your system for a global drush_language: e.g. do updatedb &amp; locate drush_language. Then delete any folders called drush_language.
Go back to your drupal root folder an do: drush dl drush_language --destination=./sites/all/modules/