Share your experience!
I thought that the T9 language is changed with system language, but I just tried Arabic and the keyboard remained English.
But in general, there's no specific option to change the language in Settings. I suggest that you try a 3rd party app, for exaple this one (supports Russian):
https://play.google.com/store/apps/details?id=intelgeen.rocketdial.trail&hl=en
You can also suggest this feature to Sony directly:
You can add extra writing languages from keyboard settings and you can use two of them.