@ArneATK. I cheat, but only a little :). I do this by adding code to force the UI language I want to test. For the video, I used a WPF app, but this also works for Windows Phone (Silverlight) apps as well.
Here is the code I used. Note: The debugger.IsAttached test. This saves me from shipping a forced UI language.
private String overrideLanguage = "fr-FR";
if (Debugger.IsAttached == true && overrideLanguage != null)
CultureInfo ci = new CultureInfo(overrideLanguage);
Thread.CurrentThread.CurrentCulture = ci;
Thread.CurrentThread.CurrentUICulture = ci;
Hope this helps.