Pick the desired language from the populated list and click Add.Once in Language and Region, add a new language by clicking on the + icon.This can be found on the top row of the System Preferences window. Once in System Preferences, click on Language and Region.Click on the Apple Menu on the top left of the screen.If this initial option needs to be changed later on, it can be done so through the system preferences application. On the initial start of Mac OS 10.9, the operating system will prompt for a default system language.