When a person speaks two languages we call them bilingual, when they only speak one language what do you call them? American.
Now that joke may not apply to you but odds are that you don’t speak as many languages as your Macintosh. And if you don’t happen to speak Azerbaijani, Breton, Croatian, Esperanto and/or Tongan then those languages are taking up space on your hard drive. Even if you remember Spanish, French, or German from high school you may not ever plan to look at an application with the user interface set to that language. If you could delete those language files then you would save disk space. How much disk space you will save will depend on how many applications you have installed and how many of those applications come with a multilingual interface. On my computer I saved 2Gb of storage space. A friend saved 4Gb by deleting those files.
One tool that makes it easy to delete the unneeded bulk of both language files (as well as binaries compiled for a processor chip that you computer does not have) is the free application Monolingual. Select what you want to keep and what you want to delete and then press a button and what. Of course, before you do something like this a backup is always recommended.
Editors note: I will second Chris’ recommendation for having a good full backup prior to running Monolingual. You may remember a time when I recommended Monolingual on the Maccast prior to them adding Universal support. Needless to say some Intel Mac owners were not too happy. The application is Universal now, but still caution is always smart when modifying your system at this low a level. I personally avoid the need to use a tool like Monolingual by doing a custom install and only loading the desired dialect when I re-install OS X (which I will do when Leopard is released).