How to set the default language.

I noticed that the Spanish translation has been completed 100% but when I try to install from the back end it shows it at 57%
Spanish Maintained by: Hertak · mrvallejo · es Not Installed 57 % 2013-10-02 17:11:44 5

I tried installing it it says it installed but it still shows Not installed.

Also how do I select the language for the front end. I dont see anything in the documentation or menus to set language.