Click the "Change" button or use the slider to change the key. Click "Encrypt" to encrypt or decrypt the message.
This encryptor should support English, French, Spanish, and Greek. If you're using Greek, the tonos has to be it own character (΄α, not ά). Also, make sure you're using straight quotes "like this" or angle quotes «like this» and not curly quotes “like this”. I've only noticed one problem so far. It's entirely symmetrical except: At Key = 20, "k" becomes "0" for some reason. At Key = 21, "l" becomes "=". At Key = 19, "n" becomes "5". I assume that there's something like this for every key, but I don't know how to predict it.