Mule (or MULtilingual Enhancement to GNU Emacs) was
a multilingual editor based on GNU Emacs. Now most of multilingual features
of Mule are merged into GNU Emacs as MULE (multilingual environment).
You may still need Mule, however, if you want to have your text written bi-directionally (for example, when you write a English AND Arabic article.)
With Mule, you can edit and handle multiple scripts with multiple character sets, and the wonderful thing about Mule is that you can add your favorite or indispensable script as you like.