To change the language in Mac OS X
- Click on the Apple icon in the status bar and select "system settings...".
- In the top bar and then "language & Region".
- Select under "Preferred languages", the Plus and select the new language in the menu.
- You will now be asked if you want to the language as the primary language of use. This may require you to confirm.
- Close system preferences and restart your Mac to apply the Changes.

Mac OS X: how to change language
The manual applies to Mac OS X 10.9.3, other versions may differ. You want to your Mac completely to a new language setting, you should also change the keyboard layout.