Be very careful if using Monolingual. Read the documentation very carefully. Be aware of its default settings, some of which have nothing to do with removing languages, and can do some quite unexpected, unwanted things. It won't save you much more than 2GB and it will take forever to run.
If you need to make room on your drive, there are much bigger files in your Home Folder, such as movies -- one movie alone might take up 10GB -- you should move either to an external or simply trash.
You could get OmniDiskSweeper to see what's taking up room.
http://www.omnigroup.com/products/omnidisksweeper/