Documentation
FastWPF supports you further still, in the localization of the application. By default, the system language is used, so the XAML specific language settings are set to the system language. However, if you change the language, the preview will be loaded with the language you specified and thus, in the case of a localized application, the texts will be loaded as well.
It is irrelevant whether you provide your texts with RESX files or XAML files, even completely independent developments are supported, as long as you consider the XAML Language and the Thread Language.