Version 2.x.x
In this version there is no way to change your language from within Fruit Ninja.

The game automatically mirrors whichever language is active on your device, unless your language is not supported in which case it should revert to English. 

Version 3.x.x
In this version language can be changed in game. At the main menu, tap on the button at the top right of the screen (the button with 3 horizonal lines) then tap on settings button, then tap on languages button.