Hello David Pusch,
Thank you for using our services. The limitation for this feature stems from the architectural design of the localization mode and unfortunately there is no way around it. Pages in Sitefinity are represented by to objects a PageNode (containing ulr data, properties such as description, etc) and PageData (holds the content of the page. When you localize a page in sync mode you basically have the same page node with localized page data. However in split mode you create two different page nodes and page data objects. Since however Group/Redirect pages do not have page data and exist only as a page node they cannot be split or in the same line of reasoning a standard page cannot be converted to a group page when it's in sync mode and has a translation in another language, as this will be a different page node.
I hope you find the above information useful. Please do not hesitate to get back to us if you need some additional information, or have further questions regarding the above functionality. We'll be glad to assist you further.
All the best,
the Telerik team
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