1. Wotlk is the most populated expansion here. There are no WoD realms.
2. List of realms is on main page.
3. Peak times are day/evening for Europen time zones but there are still many people online at other times. You can check information page.
4. On wotlk realms all raids are available, except on Lordaeron, which is progressive realm and is currently at Ulduar. All battlegrounds except Isle of Conquest are available and that one is also most likely going to be enabled again soon.
5. Lordaeron - x1 rates.
Blackrock and Warsong - instant max level pvp-only realms.
Other realms - x7 experience and x5 gold.
6. Yes, on all realms except Lordaeron.
Currently Lordaeron has much higher population than the other wotlk realms but on May 20 Deathwing and Ragnaros are going to merge.
Most are not from English speaking countries but it's the main language for communication between players so most of the active players are able to speak English.