I have logged a task to automatically add lang attribute to HTML tag of all public pages (#105852). At the moment you can added the attribute manually to your masterpage. If you have more than one language you will need to create masterpage for each language.
the Telerik team
Do you want to have your say when we set our development plans? Do you want to know when a feature you care about is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items