In Windows, go into Control Panel then click "Clock, Language" and then "and Region\Language". Click "Add a language" then select the language you want. On the bottom-right hand side of your screen, you should see "ENG" or "EN". This represents English. To change to another language, click that symbol and select the language you want (the options will be the ones you added in Control Panel).

What language do you need?

Windows 7 and 8 .. I will change it to Chinese.

Windows or Mac? What language?