Save Space -- Make Your Mac Monolingual
The files you're looking for have the file ending .lproj but don't do a willy-nilly search and destroy for them -- if you accidentally chuck the English (or Br_Eng, which is British English) you may be locked out of your Mac. Also, the system software requires a specific set, and there are some programs (like MS Office in particular) that will bug out if you delete them. So the best way for most users then is to open Applications, select an Apple app and then right-click (or control+click) and select Get Info. In the window that pops up go to Languages. Then select all but the English (or Br_Eng where applicable) and hit remove; you can select several at a time by holding shift down and clicking on them. As you do this, notice in the top of the window how the file size of the app is cut nearly in half in many cases! Now try running the application. If all is hunky-dory, empty the trash and move on the next app.
More advanced users might try Monolingual, a free, open source app that automates the process, however be very sure you know what you're doing before fiddling with it or you can brick your Mac with a click of your mouse by deleting system fonts or other files the OS requires. You have been warned.