Hi Christian,
Thank you for contacting us.
First of all let me apologize for the late response.
About the missing french UI regardless the existent french resource file.
In fact this is a bit wired case. If the localization rely on the current culture then the application have to declare which cultures are supported. Because of this you should add the supported cultures in the project file of the Silverlight application. For example you can follow the steps below:
- Open your solution into the MSVS.
- Open the Solution Explorer.
- Find the project of the Silverlight application and unload it:

- Open the project file in Text-mode:

- Find / Add the SupportedCultures element and fill your cultures:

- Save the project file and reload the project:

- That's all. Just rebuild and run the application.
As this is not so flexible we had implemented our localization manager. Please preview the
Localizing RadUpload for Silverlight with a Resource file article. In the attached example we do not care about the current thread culture - we just replace the resource manager in charge. This way we are avoiding the code about supported cultures inside the project file.
Please give it a try and let us know if you have more questions.
Greetings,
Ivan
the Telerik team