In the Captain America movies, some of the actors perform dialects well and others do not. As an actor, it is best not to imitate another actor, who may not have it right.
Here are some insights for performing dialects for movies. First, perform the character and the action. Understand that the dialect is only one aspect of the character's life. The actor must consider many factors concerning his/her character: age, education, personality, social class, health, and much more. Most importantly, the actor must consider the character's objective and obstacles, as well as the character's relationship to other characters and factors in the story.
Further, performing dialects involves attention to the focus in the mouth of the dialect, its musicality, and adjustments of sound of consonants and vowels. Start with studying the standard idiosyncrasies of the needed dialect. Then if the character is set in a specific region, study the regional dialect, which will have more specific adjustments. If the script is historical, study the style of the regional speech, during that period
Avoid imitating other actors, because even notable actors in well-known movies get it wrong. For example the stereotypical way many actors pronounce the "th" in German is with a "z" replacement. "Zer" (their) pronunciation for "th" often would sound better if they pronounced the "th" like a "d" so that "der" (their) pronunciation sounds more realistic. In the Captain America movies, some of the actors perform dialects well and others do not. It is best not to imitate another actor, who may not have it right.
Work with a knowledgeable dialect/dialogue coach. Learning and performing dialects is not difficult. With the help of a coach, actors can master dialect quickly and effectively. Once the coach is familiar with the script and character, she/he can pinpoint the needed dialect and assist the actor to perform it believably and clearly.
Once the actor has determined with the help of the coach how to perform the script, it is most important to keep the choices and performances consistent.