In the Sharing tab for Languages, add the option to make "Offline remote translation synchronization". This should be done by storing a list of the local translations made (in case it doesn't exist) and sending it by Cron/Update operations, or through a "Manual Sync" button. The main advantage of this is to make it faster to translate (the synchronization process takes around 1-2-several seconds that can make a difference when you are translating several strings).
BTW: Being Portuguese, I want to configure the remote localization server as Portuguese (pt-pt). However, when I try to change the server URL from http://localize.drupal.org to http://localize.drupal.org/translate/languages/pt-pt (which I think it's the right one), I get the following error:
Invalid localization server address specified. Make sure you specified the right server address.
What is wrong? I've the key-API defined!