Originally Posted by Dark Raven
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.
When I speak English I don't have two modes where one is that I try to sound like a native speaker and the other is me having a Swedish accent. I only have one mode and that's me trying to speak English as it's supposed to sound in one of the native speaking regions. A language isn't just the words and the grammar, there are proper pronunciations as well and it's being taught in schools.
Me returning to a default would be actually speaking Swedish, not speaking English worse than I normally do.
For Natasha it's even more normal for her to speak with an American accent since if you move away and live in an English speaking country you will start to adapt that accent, especially as a non-native speaker. That of course goes for all languages when you move to a native-speaking region.