There are many movies and TV shows that depict characters from historical eras who would not speak English, but do for the sake of the show’s audience. In those cases, they tend to use an old English dialect to mimic how the characters would have spoken in their own language.
For instance, if a movie is meant to take place in ancient Japan, the characters will speak in an English dialect, using English slang and such, even though the people of that era spoke ancient Japanese and didn’t use the same slang or have the same accents as the characters in the movie.
Another example would be a show like Spartacus. The characters are either Roman or from tribes in the surrounding areas. They likely would have spoken Latin or Greek, but the characters speak English in a way that sounds ancient, but is clearly not how they would speak during the Roman Empire.
Is there a word for that?
TV Tropes calls this The Queen’s Latin.
This trope is used in film and television fiction set in the past (or a fantasy counterpart culture heavily based on the past) where characters speak with British accents, even though the film is not set in Britain and the characters are not British.
The most common convention . . . is to employ formal English parlance. Depending on the antiquity of the era portrayed, the characters may lapse into a form of Early Modern English, or its contrived cousin, Ye Olde Butcherede Englishe.
Ye Olde Butcherede Englishe, of course, is a related trope involving the attempt to make English sound old-fashioned or Medieval, without actually doing a good, authentic job of it.
The use of plain ol’ American English (or any other language of the target audience) by characters who should speak some other specific language is probably an example of Translation Convention or possibly Aliens Speaking English (that’s specifically space aliens, not foreign nationals).