D/P are bigger than R/S though, so would likely have a longer translation period.
Ok, look at it this way:
DS cards are half the width of a GBA cartridge and also half the length, yet the DS cards can play with graphics that surpass the N64. Now, considering that they're still using sprites for a majority of the game, that leaves alot of memory left for all the text. Now, since it's possible to make a game better than N64 graphics in just half the size all around of a GBA cartridge, it shouldn't take that long to translate it. Not to mention that sometimes they'll translate as they program it, or at least make a text dump that's been translated. Pretty much, the earliest we can expect D/P is Jan. and the latest in April (and the biggest reason why we have to wait isn't because of translation, it's because that companies will sometimes work on several other projects and games, and instead of having 5 different games released by the same company at the same time, they'll break it up into little bits: We still have Pokemon Ranger and Mysterious Dungeon heading our way, both of which aren't coming to a DS soon until around the same time Diamond and Pearl hit the shelves in Japan. Plus, there's the other version of Mysterious Dungeon that they plan on sending onto the DS so that both versions are on the same platform instead of how there were two different versions: one on the GBA and one on the DS. It's mainly the marketing reasons as to why we don't get it soon. Plus, there's also the shipping. It will take at least a week to ship ALL of the copies of DP to America and all across the world to the respective branches in each country. Once that's done, it will take at least a month for them to ship all the copies of Diamond and Pearl to each store, and that one month is at the earliest.
When it comes right down to it, translation isn't the reason why games are often 4-7 months apart from the US release and the Jap. release, it's the marketing and shipping. In fact, by November the games should already be in the hands of Game Freak's branches in America and everywhere else, and it'll probably take them awhile to translate the copy that was sent and then to make more copies that are already translated. Those final copies are what go to shelves, which as I said before, take time to get to. Once after all the orders are complete, they ship the games out, and then record who gets the order and when. It's just like in the movie business, a release date is only finalized after every store has their order and are set for selling.