To Install Drupal in different languages:

- Download the new language package from http://drupal.org/project/translations

- Extract the downloaded file (nl-6.x-1.tar) in the drupal root directory (overwrite)*

  *the directory structure in the zip file is identical to the root directory
  -modules
  -profiles
  -themes
  except for a new directory translations in the separate directories which will
  contain the translation files.

  example modules\aggregator\translations

- Now go back to the initial installation www.example.com/install.php

- Select the new language

Changing the language of the user interface is also possible after installation
- make sure you selected the Locale module
- Go to Administer | Site configuration | Languages
- Add the language you require
- You will now be able to select a different default language