Open Settings > Time & Language > Language > Add a Preferred Language.
Type “Russian” and select it from the list.
Select “Install Language Pack,” choose any or all of the features you would like to install (such as text-to-speech and handwriting) and click Install. Do not choose “Set as my display language” unless you want Windows to be in Russian as well.
Once the Russian Language Pack has finished installing, you can hit the Windows Key + Space Bar to toggle languages from your default language to Russian.
For the option to use the on-screen keyboard, follow the instructions here.