I now have the 'english to 1337' part done very well, and it works 100%, thank you all for your help
I'm going to start the reverse of it, however it seems to be somewhat problematic.
Since 1337 letters can consist of more than just one character I can't simply use a loop and go through each character individually and replace them as needed, I need to scan the strings. Since there will be a space between each 1337 character I can easily tell the computer when one letter stops and another starts, however I have no idea how I can tell the computer to replace one 1337 string with an english character.
So would anyone have any ideas how best it would be to approach this problem?