1. You can use Active Directory
2. There is no limitation to the versions you may have.
3. It depends on where you want to set the language - for a module or Generic Content control. In both the cases you need to map the external templates and change RadEditor's declaration in them to support the language you want.
the Telerik team
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items