Your English is great! You may even meet someone from Germany there. All the buildings are great at CN. Everyone has his/her favorite building so don't be surprised if you get many different answers. CN is compact so you are never far away from anything on the resort. You will have a wonderful time!
We just stayed in building 1 which is right next to the beach grill. I didn't think we'd like being so close to the grill, but it was actually a fantastic location. Close to everything - pretty much right in the middle of the resort. Building 6 is great but they were doing remodeling when we were there in July. If you want to be close to the A/N beach, go for Building 9.
You won't go wrong with any building - you're at CN!
Building 6 has been rumored to be currently receiving some work.
It will do no good to email with a room request unless you are changing room category. You can make your request once you arrive at the resort and IF it is available they might be able to honor your request. You won't find a bad room anywhere on the resort.
I wouldn't worry to much about mosquitos. The resort is generally not bothered by mosquitos and there is a gentle trade wind that keeps the air moving.
It they or sand fleas are of major concern bring some Skin So Soft which has the ability to repel them. Personally I drink more rum and if they dare to bite me first I bite them back. The rooms have AC and ceiling fans which do a great job at keeping them out of the room.