The Meaning of Life, the Universe, and Everything.
Well, for starters, you'd have to go into the minecraft.jar and open the lang folder. Then you would Ctrl+C a file (like the file en_US, etc.) Then you would paste onto the desktop, or anywhere else. Then you would open it with Notepad++ (normal Notepad doesn't work as well, don't use it) and edit stuff. After you're done, rename the file, then drag it back into the lang folder. That's it! Now you can go to Languages and choose it! I hoped that helped.