Localization of WPF apps has always been a tricky issue... Theres now a loc doc that can help you in localizing your apps. Its a pretty long doc but is very informational. Do read it and provide feedback.
Thanks Lester for this link !
I particularly like the part about Resx localization. IMHO, the native WPF localization system is absurdly complex, and Microsoft provides no tools to help in the localization process, so Resx are an interesting alternative...
Actually I had already developed my own localization system using a markup extension and a ResourceManager, but the one described in this article seems much more complete, and addresses several of the issues I encountered