Padt wrote...
I once saw the idea thrown around that perhaps, regardless of your Shepard's background, there's only really one "Shepard" family in the Mass Effect universe: Hannah Shepard's. And maybe that family is a famous military family, enough so that a criminal like Earthborn Shepard would decide to use their name while enlisting for the Alliance in order to hide his/her identity and crimes from the recruitment officers.
If you wanna implement that little headcanon, Shepard could theoretically be from anywhere in the world. The name is false, so it doesn't necessarily reflect his or her background, and maybe the accent was picked up while in the military or from whatever language tutorial Shepard used to learn English.
Mine actually chose it as a name in favor of her old nickname (Lady Die; her name at the time was Diana, though even that wasn't her birth name) after a rival gang leader responded to her more-than-usually-charitable actions interfering with his operations called her "a regular ****ing good shepherd." She rapidly took the pseudonym Good Shepherd and liked it enough that she slightly altered the spelling for her actual last name when she chose a legal one, a necessity for signing up for the Alliance. This is also why no one uses her first name, because she likes the last one so much more.
Her actual birth name, Sinead, she doesn't find out until after ME2, and then she still doesn't really use that.