Translators need to install SVN (Subversion) on their system. Just do apt-get install subversion
Debian Installer contributors who want to work on the code of the installer (so not just translation work), additionally need to install git and myrepos by apt-get install git myrepos
Users of non-Debian Linux distributions will probably easily find installation sources for Subversion and git in their distribution packages list.
For MS-Windows users, a good SVN client is TortoiseSVN, which is free software. It can be downloaded from http://tortoisesvn.tigris.org/.
Successful attempts of using TortoiseSVN for Debian Installer commits have been reported. The only trick is to feed the checkout URL alone to the "svn import" command of TortoiseSVN, without the leading "svn co" part.