I'm installing Ubuntu (12.04) right now.
I wanted to know if additional language packs have to be downloaded because my Internet is very slow. I chose English as my language.
Your ISO does not contain everything you need, instead it holds most necessary things.
There is a guide to install a language pack here.
In short: you don't need to install additional language packages. However, spelling, grammar, and punctuation differences between English should be considered before deciding to not install additional packages.
Since your main installation language is English, you don't need to worry about additional language packages, unless the application/package is released not in English.
If you can download packages outside of your PC, you can use this script and execute it on Internet cafe's or your friend's PC to obtain additional language packages.
sudo apt-get <<<apt-get command and options>>> --print-uris -qq | sed -n "s/'\([^ ]\+\)' \([^ ]\+\) \([^ ]\+\) MD5Sum:\([^ ]\+\)/wget -c \1/p" > script.cmd
Question related: How can I install software or packages without Internet (offline)?
I think it's necessary to download. Check here to find a nearby server to add in your