http://drupal.org/node/5961
You cannot import a translation if open_basedir is set, ie. you are on a multiuser hosted server. As you can see from the node id, this is a very old bug, and should have been fixed long time ago. Please help getting it in!
This patch is now updated, and started to being tested on the core modules (locale import, upload module and user pictures). Please help test in more environments, and signal your interest in making Drupal work in hosted environments. Especially with our translations.
Goba