The translations for the Ubuntu installer come from two different sources:
See this link for now:
Ubiquity is the graphical installer for Ubuntu.
It consists of two templates:
ubiquity-debconf - It contains the bulk of translations for the installer
ubiquity-desktop - It contains the itle and description of the "Install" desktop icon displayed on the desktop CD.
Here is an example on what it looks like: Ubiquity translations in Ubuntu Maverick
Unlike other packages in Ubuntu, Ubiquity translations need to be manually exported from Launchpad and integrated into the package before users can see them. Translations are exported from time to time during the development cycle.
To be sure when they are exported, you can check on the change log of the particular release (search for the keyword 'translations'), or on the source code. Examples:
For more information, you can ask on IRC at the #ubuntu-installer or #ubuntu-translators channels on Freenode.