Nehrim is now available with English subtitles but continues to use the German voice acting. This may put a some people off, but they will be missing some truly great work. The world of Nehrim has nothing to do with Tamriel or any of the ElderScrolls universe, it is an independent work and you can expect to spend a good 50 hours there. The world and it's dungeons have all been hand-crafted for uniqueness, but it uses mostly the same textures and models from the main game.
Having spent a good 100 hours in Oblivion, and taking years to do so, I don't know if I will head back there anytime soon. The similarities to the original game are too numerous to make it feel different enough to spend another 50 hours there, but I will definitely be downloading it for the eventual future play.