I've looked into that in depth for another user; the language packs that are being downloaded are for your selected language only. I traced which files were being downloaded during installation (with sudo ngrep "POST" tcp and port 80), and indeed only language packs for your desktop environment, Firefox, Thunderbird, and such, are being downloaded. And only for one language. The total size of that is a few megabytes. Which however does mean that if the language packs for all 80 or so languages would be included on the ISO, the ISO would be a few hundred megabytes larger. So the balance here is; download a few megabytes during installation, or download hundreds of extra megabytes for getting the ISO?
Linux Mint 14 Nadia / 64-bit / Cinnamon