Method 1
RadFileExplorer does not provide localization for the mentioned: Copy , Close, Delete , "No records to display (the grid with no
files)" texts and the only solution is to override the texts by using JavaScript as or server-side properties of the controls (such as RadFileExplorer1.Grid.MasterTableView.NoMasterRecordsText = "no text").
For further information heck the following forum thread Some texts are hard to localize. Method 2
The RadFileExplorer control uses the same localization mechanism as RadEditor (.resx files in the App_GlobalResources folder). If you want to change the localization strings, you need to copy the RadEditor.Dialogs.resx file in the App_GlobalResources folder of your application and edit the strings inside.
For further information check the following forum thread Localization.