I had similar problems with Telerik Editor when using non-English locale (en-US is enabled for the portal, but user has a different preferred locale).
Creating a set of resource files for my locale in /Providers/HtmlEditorProvider/Telerik/App_LocalResources (actually just a copy of English resource files) helped to solve some of the problems inside editor area, but dialogs in Telerik Editor, such as  "Image Manager", "Flash Manager", etc., are still non-functional -- they appear with an empty list of folders and the list of files contain some items, but their names are blank. And it is not possible to insert an image.

Has anyone experienced the same? Or do I need to create some additional resource files in other folders that are connected with RadEditor?

Following Luiz's advice I cloned all the files in the Providers\HtmlEditorProviders\Telerik\App_LocalResources and  Providers\HtmlEditorProviders\Telerik\Dialogs\App_LocalResources creating an en-GB version of the files e.g. SaveTemplate.en-GB.resx. Now when I enable en-GB and disable the en-US it works fine including the image and Flash managers. Thanks Luiz

I also have the same problems with the Telerik editor - see attached image.  But I only have the en-US language installed.  Can someone please confirm what .resx files are required by telerik so I can check that I have got the default en-US versions? How can I solve this problem?  I had to install CKEditor instead from codeplex.

Thanks

Oscar

This is a screenshot of what the editor looks like, and my admin languages and host languages settings:

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