This November’s Black Widow will help fill in the final pieces, but we know a fair amount about Natasha Romanoff’s backstory in the MCU already. Originally a top Russian agent for the KGB, raised as part of the relentless Red Room program, she came onto the radar of S.H.I.E.L.D. who sent Hawkeye to assassinate her. Clint Barton spared her life, however, and she defected to the U.S., eventually joining the Avengers yada yada. But one big question we don’t have the answer to is: why does Nat have an American accent?
Natasha was born and bred in the Mother Country, only moving to the States as an adult, yet she possesses no hint at her Russian background in her voice. Obviously, from a real-world point of view, this is because it’s better to have Scarlett Johansson speak with her natural accent rather than attempt to maintain a Russian one. Marvel later found that out with Elizabeth Olsen’s Scarlet Witch, too, when she dropped her own Sokovian accent after Captain America: Civil War. But how does it make sense within the universe?
Well, perhaps the answer can be found in Agent Carter. Peggy Carter’s nemesis in the spinoff show is another Black Widow, also trained up in the Red Room, who goes by the name Dottie Underwood. A visit to the facility she was brought up in reveals that Dottie was trained to speak impeccable English with an American accent in order to infiltrate the U.S. Even when the jig is up, she continues to use her adopted accent. So, clearly those agents who are specifically bred to blend in in the United States essentially have their natural accents erased somehow to make for perfect, unbreakable cover.
This would explain why Natasha’s family, Yelena (Florence Pugh), Melina (Rachel Weisz) and Alexei (David Harbour), do not share her Americanized speech patterns and sport their own thick Russian accents, as seen in the Black Widow trailers. Note that Natasha doesn’t even slip into a Russian accent when around them, providing further evidence that her accent change is permanent. Perhaps through brainwashing techniques, which we know the Red Room dabbles in.