This can be done by clicking on the Apple Menu and then Restart.
Only the following languages are fully supported: Arabic, Chinese (Simplified), Chinese (Traditional), Czech, Danish, Dutch, English, Finnish, French, German, Hebrew, Hungarian, Italian, Japanese, Korean, Norwegian, Polish, Portuguese (Brazilian), Portuguese (European), Russian, Spanish, Swedish and Turkish.
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.