I know there's a program out there that's supposed to take care of this, but I used it once and it screwed my computer up. I don't know what it actually did under the covers, but I knew what I wanted to do (remove language packs!). Here's what I came up with using Terminal.

Applications, utilities, etc., have multiple-language support built in, often for dozens of languages. OS X itself has a ton of these language files, too. The ones I want to keep are for English language only, and I'd like to nuke everything else.

Here's how to do it for your Applications folder only:

1. Take note of your current available disk space ("Get Info" on your hard drive icon, for example)
2. Open a Terminal window and paste the following: cd /Applications; find . -name "*.lproj" ! -name "English.lproj" ! -name "en*.lproj" -exec rm -rf {} \;
3. Take a look at your available disk space again, you should see a substantial space savings.

Translation of Step #2: from your Applications folder, find all things that end in ".lproj" (those are the language translation folders), but exclude "English.lproj" and anything starting with "en" (this catches "en_US.lproj", etc.). For everything that makes it past that filter, delete them immediately ("rm -rf").

I ran this on two Macs at home and freed up 1.1gb on one, and 1.5gb on the other. Don't forget to check your free disk space before you run it so you'll get the satisfaction of seeing how much space is available when it finishes.