Originally Posted by Mjölnir
For me complete mastery means that you sound like a native. It helps being a spy a lot as well when you need to take on different identities.
If that's what she's doing, then during her down time when she's not undercover, why not drop the accent? Why does she need to keep up the charade? If she were to take on a German or Swiss identity, does she still even think in those languages and continue to speak like that, even if say, she were with Hawkeye and they're by themselves and no-one else is listening? She's not undercover within SHIELD as far as we know.
Anyone who has "complete mastery" over a language never does that, even if they sound like a native when they're speaking it. Natasha sounds as if she's been so subsumed into this "American" identity that she no longer seems Russian, and even if she were to go undercover as a different nationality, she might still slip back into American as if it were her default setting.