In the languages section of your System Configuration, you can select the main language of the system and any additional languages you wish to make available to users.

Language - the first field at the top of the screen is where you set the primary language of your system.

Manage the additional languages -  you can also set any additional languages that you would like to appear. To do this click on the languages in the right hand column Available Languages. Drag the ones you want into the left hand column, Selected Language(s). Once you have added them in, save your changes.  
Please note, this list is the current list that Xperios supports.