How to change your Windows 8 language
This is not a new feature: in Windows 7 Ultimate, all language packs were also available. The novelty here is that this functionality is available to all Windows 8 versions – including Windows RT.
From the Control Panel, under “Clock, Language, and Region” click on “Add a language”. In that windows, click again on “Add language”, follow the instructions, and then re-arrange your language order.
Source: Change your language or keyboard