Some WordPress translation teams choose to use launchpad.net as their translation tool. The WordPress project on Launchpad is here:
So let's say your WordPress localization team wants to use Launchpad. You've got your localization team set up. What next?
1. Make sure each of your translators has a Launchpad account. On the front page of launchpad.net you'll see a very prominent link to where you can register. Try searching for your name first though. It is possible that the system already knows about you, even if you've never actually registered yourself. In that case you should be able to claim the existing account.
2. Register a team on Launchpad. You do this from the same "Register" section of Launchpad. If you translate to French (language code "fr"), a good name for the team would be "wordpress-fr" and "WordPress French Translators" would be a good display name.
3. Once you have created your team, you'll see an "Add member" link in the actions menu on the side of the page. Use that to add each of your translators to the team. Only members of this team will be able to edit the translations.
4. Sign your team up for the WordPress translation group on Launchpad. Only members of this translation group will be able to access WordPress translations in Launchpad. It is managed by Nikolay Bachiyski; approach him directly or on the wp-polyglots mailing list, and ask him to add your team to the group. Give him the exact name of your team in Launchpad, and the language that you translate to.
5. Start translating! If you go back to the WordPress project page, you'll see several release series. Pick the series you want to work on and click on the Translations tab. You'll see options to upload and download translations, and you can click through to your language to edit translations in the browser.
To share your translations with the world, use the "Download translations" option. This will produce regular gettext translation files that you can submit to the WordPress site.