Indeed the problem reproduces both on your site, and on a local project using version 1210 (SP1). The problem is related to using multilingual mode - as a workaround try switching it off if you're not using it and the problem should be fixed. Also, please have in mind, that testing the same issue with our latest internal the problem did not reproduce, meaning we have introduced a fix for this and it will be available in Sitefinity 4.1 release next week.
All the best,
the Telerik team