Can you confirm you have all the needed files present in the App_GlobalResources folder? You can see a list in the attached screenshots (and I am also attaching the files themselves even though you can take them from the RadControls installation).
In order to localiza the the given English culture the default files must be present (without culture suffixes) and the desired culture must also be there (e.g. en-GB). Could you make sure the culture is set to a supported value (e.g. en-US, en-GB, etc.) and not a value that does not match the resource names?
If you cannot replicate the issue on your end can you have the client perform a check for the resources that are loaded in the browser and if some are missing to see which ones? If a webresource request fails this blog post
can help determining which one it is. Firewalls and proxies can also block requests.
If you can take a look at the server logs it is possible that the server itself cannot access the files for some reason (application pool recycle for example).
If this does not help - can you share with us a project that is causing the problem so we can investigate the case?
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