If you told my dog to "shut up" i'd get offended. As twisterdog said, dogs can't understand English -- But, I can. So, it would be the same if you told my kid to shut up.
I would like to avoid a misquote, if I can at this late date. When I said dogs can't understand English ... I was using that to back up my argument that it is perfectly ok to tell a dog to shut up.

My main point was this: it is the dog owner's responsiblity to keep their dog quiet, to keep their dog from being a nuisance. If the dog owner is doing his/her job, training and controlling the dog, then no one else will ever feel the need to tell the dog to "Shut Up!" or anything else.

If the dog owner is in the house, watching Oprah, and the dog is running up and down the fence, barking at the neighbors in their own yard ... then, IMO, those neighbors have every right to tell the dog to "Shut up!", to tell the dog's owners to keep it quiet, and to report him/her to animal control if it continues. Even worse ... if the dog owner is standing outside while the dog is barking, barking, barking!

Same thing with my kid, frankly. If my child is being overly loud and unruly, I need to control him and make him be quiet. If I am not doing my job, then I think the person in the booth behind me in a restaurant, at the next table in a library or the pew ahead of me in church has every right to turn to my child and say, "Shhhhh!" and/or to turn to me and say, "Could you please keep him quieter, I'm having trouble hearing." No, they shouldn't say, "Shut up!" because my child understands English ... my dogs don't. Therein is the difference I was trying to point out. But either way ... it is the dog owners/parents responsibility to handle the situation BEFORE anyone NEEDS to say anything. Just MHO.