1. It's not official but is based on official MIUI. So as close to official as you can get. There are stable versions and weekly versions just which are based off official miui.
2/ yes you need to unlock your bootloader to install it. You don't need to root it. You can use superSU if you want, it's optional.
3/ I'm not sure, there are many extra locales/languages added. Check a rom thread where you will see features and additional languages, etc.