How to test a localized WPF application in visual studio 2012

Question

I am trying to create a localized application in C# / WPF in Visual Studio 2012. For that I created two resource files and changed one string in a (XAML) window to use the resource files (instead of a hardcoded string).

I see the English text from the resource file, which is correct. However, I want to check if the other resource file (fr-FR) also works but I cannot find a setting or procedure how to change my 'project' to run in French.

Thanks in advance.