First, get into Raptr/AMD Gaming Evolved's Preferences by either clicking the Wrench icon at the Control Center and clicking Preferences, or the Gear Icon on the Desktop Dock.
In your Preferences, you will go under the General Tab. There you will see a drop-down menu for "Language". Select which Language you would like and click "Okay" at the bottom of the window.
Changing the language may require a restart of the client.
(Note: English is the 2nd from the top.)