There really is no need for you to continue to show your selective understanding of English - that fact is well-established.
Well, I do speak Russian, but I actually read it far more fluently than I speak it. Natalia Veselnitskaya almost certainly speaks and reads English, but that doesn't mean that she is comfortable using the language.
As whichphilosophy says, she could find it useful to feign not understanding English, because that makes English speakers around her more lax about saying things that they think she won't get. More importantly, however, one should remember that she is a lawyer. Language is extremely important to lawyers, and they tend to be acutely aware of ambiguity, vagueness, and the propensity people have for misunderstanding. So she would naturally want a personal interpreter in a room with her, if she is there to discuss something important. That would allow her to listen to the English and then get a professional's take on the gist of what was being said, but in her native language. Also, with an interpreter, she would be able to express herself clearly in her native language and then monitor how it was translated to others. Too much is being read into the fact that she rarely speaks any words of English when talking about political matters.
I would not be the slightest bit surprised if she turned out to be a deep cover FSB officer with training in espionage. It is not useful for spies under cover to have a public record of employment by their agency. For example, Valerie Plame was a CIA officer, but that was only learned after the Bush administration outed her in revenge for her husband's failure to support their lies about the aluminum tubes sold to Iraq. Veselnitskaya's behavior is fully consistent with that of an FSB operative. She has been singularly focused on getting rid of the sanctions imposed on Russian oligarchs and their businesses.