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
cd debian-installer
scripts/git-setup
mr -p checkout
Beware it'll take a while, as its ~480MB.
The particular dialog you're interested in comes from:
- packages/partman-base/debian/partman-base.templates
- packages/partman-base/choose_partition/*
- packages/partman-base/display.d/manual_partitioning
- a bunch of .po files with translations to many, many languages
(The easiest way to find that is with ack-grep
...)
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.