A whole part of dealing with Russians as well as a large part of how their policies are shaped can be understood through culture. The characteristic bent in mannerisms that value "honor", "masculinity", less emphasis on the exact and more on "estimates" while working through problems etc. Most people who've dealt with the Russians will agree, that this series came across as British propaganda, badly inserting their societal structures to communicate the events flowing in another, completely different structure. The main character looked like a poor Austin Powers impersonator playing a Russian.
As an example, the Female Scientist, Ulana . . i have no problem with the producers exercising creative freedom to designate her character as the substitute for the hundreds of other scientists who worked on the project, but in doing so, wildly misrepresented the role of women in Russian society during those times.
The choice of English as a language itself . . fine. What the producers can't seem to grasp is the delivery of Russian is aggressive. You don't make statements in tense situations with the British smooth tongue. That does not serve the understanding of the situations during those times very well.
I could go on about the sheer lack of patriotism displayed by any of the main characters to the point where it felt as though it was forbidden when in reality, anyone who's lived through those times and interacted informally with these people knows the opposite is true, but i repeat myself.