I Downloaded the http://ftp.drupal.org/files/projects/nl-6.x-1.5.tar.gz file from Drupal Translations page. The readme file says to "Copy (merge) the content of this translation package into your Drupal installation root directory".
If i look at the package it has a few text files and a modules, profiles and themes folders. If I copy those in the root (so MAMP/sitename/) it overwrites a bunch of files (there are already modules and themes folders there ...) and whatever page I load on the site gives fatal errors.
Is it possible that on MAMP / Mac Unarchiver doesn't do copy/merge but actually Replaces the old modules folder with this new one?