LocalizeDictionary.DesignCulture is not workingin designtime

Question

i have added wpflocalizeextension via NuGet and followed the instructions in the Documentation section. My window looks like this:

xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"

xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"

xmlns:lex="http://wpflocalizeextension.codeplex.com"

lex:LocalizeDictionary.DesignCulture="en"

lex:ResxLocalizationProvider.DefaultAssembly="WpfApplication12"

lex:ResxLocalizationProvider.DefaultDictionary="Resources"

Title="MainWindow" Height="350" Width="525">

In my Resources.resx file, String1=english

In my Resources.fr.resx file, String=french

I know things are working because in the designer, the button correctly displays "english". When I change lex:LocalizeDictionary.DesignCulture from "en" to "fr", the designer still shows "english" and I expect it to display "french".