That's what Tolkien wrote. He didn't write the book as an American speaking English, or as a German speaking English. And there is a distinct difference. Bad dubbing in English can spoil the enjoyability of a film. I'm sure the same rule applies in other languages.
Dubbed voices are an entirely different problem.
A lot is lost. English is the most ubiquitous language on the planet. That's the problem. It would be unfaithful to the book if everyone spoke the same way. British accents are not homogeneous.