Wanda had a stronger accent in Age of Ultron because that is her natural Sokovian accent (when speaking English) and she was living in Sokavia. When she moved to the States and started living with the Avengers her accent softened, as will naturally happen when someone moves to a new part of the world for an extended period of time. People will start to adopt the local accent. That only goes so far, however, but if someone is actively trying to “lose their accent” to better blend with the locals they can basically eliminate it entirely over time. But even in those cases they will occasionally let their natural accent “slip” when distracted, emotional, drunk, etc...
Wanda’s accent softened between Age of Ultron and Civil War because that is what tends to happen with native accents when someone lives in a different part of the world but also because she was “trying” to lose it. Thus, it was even softer as more time passed when we got to Infinity War/Endgame. Inside of Westview it’s completely gone because she’s immersed in the American sitcom façade. When she was more heated and emotional her accent slipped back towards more of her natural accent. Which is entirely consistent with how it works in real life.
