There doesn't seem to be any indication that Castro spoke any Russian. For their part, none of the Soviet leaders that dealt with him (and it was more Khrushchev and Brezhnev than Gorbachev) spoke Spanish. Khrushchev in particular was a bit infamous in the Soviet Union due to his relative lack of education.

But regardless of their linguistic skills, they are more likely to have communicated in their respective national languages, with interpreters, as is considered appropriate in international diplomatic discourse on that level. People like Gromyko, with solid backgrounds in the government apparatus, knew English, but even he still always relied on professional interpreters. For a more recent example - Putin is fluent in German, but also always uses interpreters when dealing with German politicians and German press.

This approach has many advantages - for example the representatives themselves are rarely completely fluent and their linguistic mistake has the potential to cause an incident. Also, it makes it easier to broadcast the remarks of the speakers in their native countries, where seeing their leader speaking a foreign language might be frowned upon. Here is an interview with Khrushchev's personal interpreter - it's in Russian, but Google translate does a good enough job - he covers the same issue there.

As a side note, within Komintern, the international communist party, the "official" language was german. Komintern was dissolved during the second world war.