Its extremely hard to change an accent, its almost impossible to LOSE an accent as an adult, and if your native language isn't originally a branch of a language who's accent you're trying to imitate, then it is IMPOSSIBLE. You could probably pick up a British accent as a native American, but good luck speaking Russian without an accent as an American, or vice versa.
Wanda is from an Eastern European country of Sokovia, speaking a Slavic language, that is written in Cyrillic alphabet (as you can tell by watching the signs and billboards in last act of Age of Ultron). Her speaking flawless English is nonsense.