Captain America: The Winter Soldier post-credit scene, we really meet Wanda Maximoff in
Avengers Age of Ultron. Wanda and her brother Pietro are being held in a Hydra facility in the fictional country of Sokovia where Baron Wolfgang von Strucker is experimenting on them with Loki's scepter. Both Wanda and Pietro have Easter European accents, which they created specifically for these characters.
Pietro dies in the battle against Ultron, and Wanda goes on to become a member of the Avengers, appearing in
Captain America: Civil War (with her accent). But, after
Civil War, when we see her in
Avengers: Infinity War and