As we have seen, Ubuntu packages whose translations are delivered by language packs (namely, those in Ubuntu Main and Restricted), are translated in the Launchpad Translations UI.
A Quick Look
Launchpad Translations provides a straightforward web user interface for translating Ubuntu.
Note: You can also use it to translate your own Launchpad project.
Here is the URL for Ubuntu Maverick translation: https://translations.edge.launchpad.net/ubuntu/maverick
From that page, at the lower right, you can "Choose your Preferred Language" or "View all Languages".
Let's say you are interested in German, so you click "German" in the displayed table, which takes you to this page: https://translations.edge.launchpad.net/ubuntu/maverick/+lang/de
A list of translatable packages is displayed. Clicking "update-manager" takes you to this page: https://translations.edge.launchpad.net/ubuntu/maverick/+source/update-manager/+pots/update-manager/de/+translate
Here you see actual messages in update-manager displayed in the Launchpad message translation user interface. You can use this to make translation suggestions. And, if you are on an official Ubuntu Translators language team, you can select the actual translation to be used.
Launchpad and Language Packs
When Ubuntu is released, and periodically later, all translations from Launchpad are exported and converted into language packages. The language packages for your language of interest are automatically installed when you use the Language Support application to install a language.
How do Translations get into Launchpad?
Developers upload new versions of source packages to Launchpad. When the source package is uploaded, the translations it contains are uploaded into Launchpad. Those translations are used unless the specific translation was intentionally modified in Launchpad.