In Martin Scorsese's "Hugo", why did all the characters have English accents if the story is set in Paris, France? Wouldn't the characters have French accents?
Why should they have an accent?
By your own logic, they should be speaking French, since that is what Parisian's would commonly speak amongst themselves.
However, translation convention and the fact that the movie is produced by an English speaking production company and targeted at English speaking audiences, the characters speak English.
As for accents - some movies do have the characters speak with accents influenced by their setting, but not all movies do. The problem comes when actors can't properly affect the accent and it can come across as a ridiculous pastiche. Hugo is played by a young English actor who, while quite talented, may not be up to the task of a good French accent.
So, to keep things consistent, get all the characters speaking with English accents.