Developers need the whole source tree, which requires the use of git and mr:

git clone git@salsa.debian.org:installer-team/d-i.git
cd d-i
./scripts/git-setup
mr checkout
  

In the past, we had everything in one subversion repository. Then one day, the developer parts were migrated to git, while the translator files were left on subversion; this is why we had the project divided into two parts now. Then, the Alioth server was replaced by Salsa, and thus everything was migrated to git.