I am not quite sure what may cause the problem on your side.
By default, RadFileExplorer have built-in English, French and German localization resources. You can find information on how to implement a custom localization in the following help articles:
Using Global Resource Files
If you have followed the instructions provided in the above mentioned articles and the problem still exists, could you please open a support ticket and provide a sample project containing the resource files so we can investigate it further?
the Telerik team
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