Answer

To change the system language, add the language to the system and set it as the primary language in [Control Panel] > [Clock, Language, and Region] > [Regional and Language Options].

*If you don't see the language you need, you can download and install the language pack before applying above settings.

Find more detail steps in the below links:
-For Windows 7: http://windows.microsoft.com/en-us/windows-vista/change-the-display-language
-For Windows 8/8.1: http://windows.microsoft.com/en-us/windows-8/add-language-keyboard