I also had some problems with DNN and RadGrid localization. Thats how it worked for me!
1. Your not allowed
to put in a culture attribute, eg. Culture="de-DE" You have to configer it in the web.config. If you have a multi language website it takes the culture selected by the User.
So again if you have the culture attribute set, it wont work/translate!
2. You have to set
the LocalizationPath to the Path, not the file, where the .resx is saved. You have to set the LocalizationPath even if the .resx file would be in the App_GLobalResource, so like this:
Be sure only put the folders Name, not the file name. You can also put the .resx file in any other foler eg. DesktopModules/[ModuleXY]/App_LocalResources
This is how it worked for me in DNN 6.1.5