The RadFileExplorer control follows the same localization mechanism as the RadEditor (.resx files in the App_GlobalResources folder). If you want to change some of the file explorer localization, you need to copy the RadEditor.Dialogs.resx
file to the App_GlobalResources
folder of your application and edit the strings inside. For example, to change the upload "select" button text, you need to edit the "Common_Select" string. See the attached screenshot for more details.
The value you have set in the codebehind is overwritten with the value from the resource file. If you need to set the value from the codebehind, you need to do it after the PreRender event of the FileExplorer, because that is when the control localization is set.
the Telerik team