In order to install additional languages in Drupal 6.6, I'm following the steps mentioned here:
http://drupal.org/node/203318
where Gábor Hojtsy stated:
1. Download langauge package
2. Extract to Drupal's root directory, where Drupal is already exctracted
3. Enjoy that all files fall into place, such as modules/system/translations/*.po
By the way, this also agrees with the info here: http://11heavens.com/building-powerful-and-robust-websites-with-drupal-6...
But I've tried this using Drupal 6.6, and I'm getting an error message.
Here's what I did:
1. First I made sure there are NO languages (other than English) currently installed, because the site:
http://11heavens.com/building-powerful-and-robust-websites-with-drupal-6...
says:
Get the translation tar ball (*.tar.gz file) from http://drupal.org/project/Translations, then move this file to your web root (in the same folder as index.php), then extract the file. Every extracted .po file (as there are many, many) ends up where it's supposed to be. The translation package has a tree structure, that of your Drupal installation. Then and only then, you add the language (going to admin/settings/language/add). At that point, the importing of all .po files is done automatically for you by Drupal.