I have just looked through this list of 10 free mac apps I have used most of them in the past but have never come across monolingual before. It removes all the extra languages that come as part of OS X and the translations from within most application. I left English, English UK, Scottish, Latin and Greek and saved about 1.4GB. it reported 1.8GB but I think this was a bit optimistic but I did not write down the free space before hand, the removed sections do not go to the trash can so the space starts to free up immediately.
Use Monolingual to save space.