In my experience, you can expect to recover between 2-5 GB of disk space by removing additional languages.
Original Post
In the wild world of Windows, installing or using localized versions of Windows and programs is a conscious decision the user has to make. Installation programs are often split into multiple versions, one for each localization offered, and using a localized version of Windows means installing a completely separate copy. If anyone has had to use localized versions of either Windows or its applications, thats a complicated procedure and at the minimum a potential time waster.
With OS X and applications made for the platform, there is only one version of OS X for all locales its installed for, and Applications are the same way. This is great for usability, and makes buying OS X or apps less confusing, but the downside is that localizations eat up a lot of disk space. If you are not using the localizations, and you know you never will, why keep them around?
Fortunately, OS X provides UI to remove localizations, they are called Languages, from an application through the Info pane on the application.
You can go through every single app and remove languages you aren't going to use, but this will not remove the extra language files from OS X itself. You can choose not to install the Languages when you install OS X, but most people use OS X pre-installed on their Macs. There is a pretty great utility, called Monolingual, a Universal binary that allows you to remove all the languages you know you aren't going to use. Here is a screenshot:
Monolingual also allows you to remove PowerPC binary data from Universal binaries if you are using an Intel Mac, or Intel binary data from a Universal binary on a PowerPC Mac, but I don't recommend it for pretty much one reason. Developers may keep certain files in a distribution as PowerPC because there performance doesn't matter and Rosetta emulation is good enough. World of WarCraft had this issue when they first released a Universal binary.