Well behaved mac applications stores their localization info on the Resources folder inside their application bundle. This localization can be deleted using finder (or you can add one to if you like). I have noticed that almost all applications from Apple (such as iTunes, iMovieHD, etc), comes with many language, and when I delete these uneeded language, I got about 1,3 Gigabytes free space. That may not seem much, but since I am running out of disk space, it helps me a lot.
Removing localization is easy, just select application, press ⌘-I (command-I, or select File – Get Info), a dialog will show up where you can remove a language. Hint: If you just want to leave English or one language: Clik on the language selection, choose select-All (⌘-A, command-A) then deselect English by holding the ⌘ (Command) key and clicking on English.
You can also use Monolingual that can do the task better automatically. It can also delete unneeded binary codes from universal applications. I can get several gigabytes free using the program.