Debian Installer is actually a bunch of different packages, in several repositories. The Debian Wiki has a page on how to get the Debian Installer source: Make sure
mr is installed, and:
svn co svn://anonscm.debian.org/svn/d-i/trunk debian-installer
mr -p checkout
Beware it'll take a while, as its ~480MB.
The particular dialog you're interested in comes from:
- a bunch of .po files with translations to many, many languages
(The easiest way to find that is with
Note that it's asked via Debconf, like the rest of the installer. So the same source drives the similar dialog in the graphical installer.