If you get on your site an error like this:

warning: file_get_contents(sites/default/files/languages/ar_bb3f64e1d3c493008e8d5eee5511c741.js) [function.file-get-contents]: failed to open stream: No such file or directory in /var/www/html/uberdrupal/includes/common.inc on line 2359.

or you found a lot of “Page not found” errors in dblog related to JavaScript translation files, probably is a Drupal Bug (if files permissions are all ok).

Indeed Locale is unable to rebuild lost js translation files, if one of these gets accidentally corrupted or lost there is no way to tell Drupal to rebuild it. This is true in all versions of Drupal 6.x, the last too.

To rebuild these files you HAVE TO use an ad hoc module (Local Rebuild Javascript), created by plach, until the patch would be applied or when you’ll update your site to Drupal 7.

Reference: Drupal.org Issue.