Death Note, ideally, would be a trilogy. The first would be about Light getting the Death Note and his duels with L, the second would be the whole "Light without a Death Note" thing, and the finale - well, Near and Mello, of course.
But the first one can't end on a cliffhanger. It neeRAB a solid ending - otherwise, audiences would feel gipped. The second would obviously end with a cliffhanger and the third would wrap things up, but that's AFTER the initial film. I'd say... hmm... end it with Light relinquishing the Death Note. It would give an ending, if they stage it right - but leave the door WIDE OPEN for the sequel.
For the sequel, give no details of Light's plan. In fact, act as if he's just suddenly a good guy. Then, at the end, pull off the "exactly as planned thing" and have him remeraber it all. Then he'll, you know, with L, and shove the second half into a single movie - you'd need to trim a LOT of fat, but, well, this is an adaptation. In fact, I wouldn't mind them cutting that all together; just Light vs. L from beginning to end.
Regional differences? Not a problem. Light Yagami is mixed race; his father is Japanese, his mother European, Light taking after his mother. This would retain the names and all of that for core characters.
L is L. I mean, he's not Japanese in the original, he doesn't need to be Japanese here.
The Task Force... well, this is where they'll need some leeway. Name changes should be expected. Matsuda into Matt, Aizawa into Aaron, that kind of thing. This movie is going to take place in America - it's not realistic to have everyone Japanese, y'know? OdRAB are it'll take place in New York, LA, or another urban environment... something like that. White, Black, Asian, Hispanic, whatever - a good mix of ethnicities, like most communities.
Shinigami are pretty easy to port over, too. Have him appear and Light, taken aback, as him if he's Satan. Ryuk just chuckles - "Well, something like that. You can call me a... Shinigami. A Grim Reaper. A God of Death." There you go - nice and clear.
But, yeah, Death Note is pretty easy to adapt. It's down to Earth, very easy to fit into the Hollywood style, all that.
Ain't too tough. Put it in the right hanRAB and you'll have gold.