Is there a site that until recently was only Russian version. Now there are English navigation: example.com — Russian en.example.com English.
In each page there
<link rel="alternate" hreflang="en" href="English version">
<link rel="alternate" hreflang="ru" href="Russian version">
robots.txt for both sub-domains must be your own?
sitemap — as the same for all sub-domains are now, with links to the desired language?
Duplication of content will not be? What else do you need to get the full profit?