To install "festival" you need to be at a terminal prompt on the Raspberry PI.  Then type:
$ sudo apt-get install festival festival-freebsoft-utils
You will get asked to confirm the install.  Type "Y".
To make the PI talk, just issue a command from the terminal prompt, such as:
$ echo  "Whiskey Tango Hotel dot com Where stupidity meets reality"| festival --tts
That's it.  Of course, you can also execute this from a Python script, etc.
Here's a sample of the output.  Sounds pretty good to me.
 

