I'm surprised that Firefox Translate includes Bulgarian but not Russian in its list of available languages. It seems to me that the latter language is far more important.
I do see from your post that there's a Russian forum (as well as the French forum of your link—pity that my French doesn't go beyond accepting the site's cookies!) But I don't know if this is related to interactive translation or not.
@Alessandro_, @drjlevi6 yes, of course you're right. I guess i'm just feeling really insecure about it.
There's great news! The new languages are: Estonian, Finnish, Icelandic, Catalan, Norwegian (Bokmel), Norwegian (Nynorsk), Persian, Russian, Czech, Ukrainian and Hungarian.