You need to make sure that the control is Languageinstalled correctly.
Tim Heyer has a blog post entitled "StringFormat and CurrentCulture in Silverlight" about this for Silverlight, so I expect the same problem to occur in WPF.
Silverlight , :
this.Language = XmlLanguage.GetLanguage(Thread.CurrentThread.CurrentCulture.Name);
WPF , CurrentThread.CurrentCulture , .