Chapter 10 Install and System Administration for FortiOS 5.0 : Using the web-based manager : Basic configurations : Changing the web‑based manager language
  
Changing the web‑based manager language
The default language of the web-based manager is English. To change the language, go to System > Admin > Settings. In the Display Settings section, select the language you want from the Language drop-down list.
For best results, you should select the language that the management computer operating system uses.