How to get list of all available keyboards on Windows Phone Mango (in C# code)?
Or how to programmatically set custom keyboard for typing?
For example I want set the keyboard to Russian language for typing in my translator app to translate it into English
You can't change the keyboard language in code. That's up to the user. But with Mango, the user can have multiple keyboards, and switch with a single button press.
You should simply assume that the user will do this.
Here you will find a list of all available InputScopes -> http://www.kunal-chowdhury.com/2011/07/windows-phone-7-mango-tutorial-26-what.html