A quote from letter #190, regarding translation of his book into other languages:

"After all, the book is English, and by an Englishman, and presumbably even those who wish its narrative and dialogue turned into an idiom that they understand, will not ask of a translator that he should deliberately attempt to destroy the local colour."

Now that is regarding translation, and in this case, the circumstances are slightly different. Instead of translating into another language, P.J. is translating to another medium. However, there is no reason to destroy the local color, as Tolkien says.

Quote from letter #193 to Terrence Tiller, regarding the BBC radio production:

"I have no doubt that if this ‘history’ were real, all users of the Common Speech would reveal themselves by their accent, differing in place, people, and rank, bu tthat cannot be represented when Common Speech is turned into English – and is not (I think) necessary. I paid great attention to such linguistic differentiation as was possible: in diction, idiom, and so on; and I doubt if much more can be imported, except in so far as the individual actor represents his feeling for the character in tone and style."

So as you see, Tolkien had already accounted for the differentiation of style and idiom. But you must remember that this was for a BBC program, and it was simply assumed that the accents would be English. Tolkien would have been shocked and very angry if anything else were used.

Tolkien's characters speak English. An "archaic" English (for lack of a better word)--one that doesn't suit itself well to any accent other than an English one.

Now, artificially assigning a particular accent (i.e. German, American, English, Chinese) to a particular group of people--as you seem to suggest--would be disastrously unbelievable. It would lend to the sort of stereotyping that we saw with the Phantom Menace. It would destroy the secondary belief which Tolkien thought so important in writing about a "fairy" world by bringing obvious elements of our own primary world (i.e. arbitrary ethnic divisions that exist on this planet) into the secondary world. And once that believability is lost, Middle-earth would become just another setting for a Hollywood Blockbuster.

A final quote, from "On Fairy-stories", a lecture delivered by Tolkien in 1938:

"That state of mind has been called ‘willing suspension of disbelief.’ But this does not seem to me a good description of what happens. What really happens is that the story-maker proves a successful ‘sub-creator.’ He makes a Secondary World which your mind can enter. Inside it, what he relates is ‘true’: it accords with the laws of that world. You therefore believe it, while the you are, as it were, inside. The moment disbelief arises, the spell is broken; the magic, or rather art, has failed. You are then out in the Primary World again, looking at the little abortive Secondary World from outside."

p.s.

As I said, if ol' Viggo can pull off an English accent and make his character believable, then it was a good casting choice, and I would have no problem with it. However, as many films have demonstrated, Americans tend to have a problem pulling off a British accent. There is a good chance, then, that Viggo cannot do it, and if so, then it will be a poor casting choice. Time will tell.