amin fear you don't need to edit script unless you want to reach some "magical stuff"
All you must to do is to prepare gfx/sfx, pack it with 7zip, zip or winrar to
setup.cab (find some of my/yener90's conversions to view the file names), customize installer via
setup.ini (tittle, developer, exe, installer archives, button positions, redist installation...), add your language by putting
isl file into
Languages folder and adding entry to both scripts under
[Languages] section, compile them and voila!
PS: Custom Installer uses some custom messages not included in official
isl files. You can translate'em under
[CustomMessages] section