It seems there is a problem with non en-US installation of the Telerik RadEditor on DNN.
There is a problem that has been lodged but an official fix is not yet out I shall explain how I managed to get it working for my cusotmers DNN5.4.2 UK English website:
Now to fix the problems with the Telerik file manager....
We provide high quality software development, database development, search engine optimisation and web design services to companies in Hampshire, Berkshire, Wiltshire, Surrey, South Coast England and the UK
eRoom and SharePoint Consultants Microsoft SharePoint Development eRoom Specialists FieldBase Business Continuity