I think this one is simple. Try going to the System Preferences and picking International. It's under the apple drop-down menu at the top left and it's the 4th one down.
In System Preferences, International is on the top row and is represented by a picture of a flag. In the International control panel you just drag the language that you want to use to the top. Then reboot. The computer should use that language now. should be in that language now.
An alernative would be to reinstall OS X and pick english.