Localize.drupal.org hosts an amazing number of language translations, and we are eager to provide the fruits to you via different download methods. The automated exports are a work in progress (see below for technical details), but we do our best to generate up to date data for you often.
Translation files are exported to ftp.drupal.org from the database as often as possible. You can navigate the FTP server from the web interface, but the root of the translation downloads is at http://ftp.drupal.org/files/translations/. You'll find downloads separated by core compatibility and project name. For example, http://ftp.drupal.org/files/translations/6.x/ lists all the Drupal 6 translation files we made available, while http://ftp.drupal.org/files/translations/6.x/drupal/ is the 6.x translation files for Drupal core itself. Browse around and find your translation files there.
Use l10n_update module
There is an experimental module to automate translation downloads from ftp.drupal.org using metadata about the modules and themes you are running as well as the translations provided on localize.drupal.org. The localization update module can be used to fetch the above mentioned files automatically and update your local database copy of translations as needed later on. You are welcome to test and provide feedback for this module, so you'll not need to manually download files in the future.
Contribute to translations and export
If you are a contributor to a translation group, you can also download .po files from the latest version of the database state. We do not recommend the use of this method on a daily basis, since it puts load on our database. We specifically came up with the static files generated on ftp.drupal.org to avoid our database loaded with exports for regular use. If you contribute to the translation efforts however, you'll find the most up to date version by exporting directly from the database.
Frequency of static file updates
As of this writing, we have about 12000 project releases and 70 translation teams, so we have about 840000 files to regenerate continually as new translations are entered. We currently look at 100 releases or generate 100 files every 5 minutes, which means we can look through all releases twice a day if no new files need to be regenerated, but if we need regeneration for all files, then we can only look at about 360 releases a day. So the two extremes is we double check each day that no new contributions were entered or it takes about 35 days to regenerate everything if all translations would suddenly be updated. Neither extremes are actually happening of course, so we are somewhere in between depending on submission activity.
Thanks for your patience and collaboration in the meantime testing the l10n_update module which will work the same way regardless of file update frequency.
If you have bugs to report about our downloads, see http://localize.drupal.org/issues for more information.