Blech ... The WPF team must have left something desired with their localization solution. Well, for what it's worth, here's what I did (to be honest, I really borrowed this idea from Jecho Jekov on CodeProject):
( ) Localization MarkupExtension. Jecho LocExtension, i18nExtension - , , , Loc. , , - . , , Properties/Resources.resx.
, , - xaml:
<UserControl ... >
...
<TextBox Text="{i18n HelloWorld}"/>
...
</UserControl>
resx UserControl/Window, WinForms, MarkupExtension, .
, , , . , , , , / . , Window/Control .