Click on the start button, go to settings-control panel. Now click Regional and Language options. Here select the language tab then click details.  Now go to settings and choose the language you'd like for the keyboard. To finish, click apply and then click ok.

From the apple menu, open System Preferences and click International to display preferences. Click the Language tab. Now in the Languages pane, drag your preferred language to the top of the list.  Restart your Mac, or log out and log back in. This will apply the language change throughout your system.