Richard Archambault on "[Plugin: Jetpack by WordPress.com] Change Confirmation Message in Contact Form"

Hi there,

It worked for me. I tried to add it in a functionality plugin rather than in my theme's function.php file, and it worked fine.

http://ryanscowles.com/2013/07/wordpress-functionality-plugins/

There are links there that should help you in creating such a plugin. Let me know if you still have trouble!