Thomson/Britannia also did some flights in this era further east into Siberia in deep winter, for "the cold". Minus 40 pretty much guaranteed. Bratsk was one such place.
I don't know whether they made a stop en route at say Moscow, and boarded a Soviet translator. the real issue with flights further into the Soviet Union at that time was not so much xenophobia, more a lack of ATC who were fluent in English. They were so staffed on one or two routes into Moscow, and into Leningrad from by the Finnish border, other than that it was an issue. Still nowadays ground control at Russian airports is Russian only, and Follow Me cars are used to lead visiting aircraft.