Before the Soviet Union collapsed, what was Russia's official ATC language?
There's surprisingly little information on this to be found via Google, at least in English, but according to this academic paper from Embry-Riddle titled Air Traffic Control in Russia, Soviet controllers operated entirely in Russian, at least within the Soviet Union itself:


  Prior to 1991, Soviet air traffic controllers and pilots flying within
  Soviet airspace were not required to speak English and the majority of
  them spoke only Russian. After the KAL007 incident, translators were
  stationed at each of the three main control centers in order to
  assist with any controllers who encountered English-only pilots, though
  they weren’t used often (Taubman, 1987).


The cited source is a New York Times article which says that "80 percent of the civilian air controllers in the Soviet Far East speak some English" but translators (more correctly, interpreters) would be used "in an emergency", implying that the standard and/or availability of English was very limited.

If you consider the Warsaw Pact in general, then it looks like other languages may have been more common. This history of Hungarian ATC (you have to love the internet!) includes this information about the 1970s:


  The regular international cooperation between Air Traffic Controls
  commenced in the seventies under the aegis of ICAO, since Hungary
  became one of its regular members. It is more than interesting that
  the Hungarian controllers had to be able to speak not less than three
  languages with the flight crews and use not more than three units of
  measurement, namely the Flight Levels and metric systems of ICAO and
  the Soviet metric system. [...] Then at last, in 1975, Hungary applied
  the full ICAO system, but the Russian language as a medium of
  telecommunication remained in constant use until 1991 when the Soviet
  military forces left our country, after the 1989 political changes in
  Hungary.


It also confirms that Russian was a requirement for ATC controllers:


  Boys in their twenties with more than basic English and Russian
  language knowledge were the target as new trainee controllers.


There are also numerous aviation forum threads about Soviet and modern Russian ATC controllers, the overall picture seems to be that there was always some English spoken by ATC but it was often grudging, limited or just hard to understand. However most commenters (e.g. airline pilots) seem to agree that since the end of the Cold War, it's become much more widespread and the general level of ATC professionalism in Eastern Europe and Russia has improved significantly.