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:

• Login to the back end server
• Navigate to ~\Providers\HtmlEditorProviders\Telerik\App_LocalResources
• Take a copy of all the .resx file in this directory
• Rename all the "Copy of XYZ.resx" files to be "XYZ.en-gb.resx"
• Navigate to ~\Providers\HtmlEditorProviders\Telerik\Dialogs\App_LocalResources
• Take a copy of "SaveTemplate.resx" and rename it to SaveTemplate.resx"
• Restart the site from the Host menu

Now to fix the problems with the Telerik file manager....