This blog post is about Windows 8 / WinRT XAML apps. It's based on the Windows 8 Consumer Preview, things can still change in upcoming versions.
There are already great resources that tell you how to create a localized app. Like the Windows 8 sample "Application resources and localization". Tim Heuer also wrote a long blog post about it.
However, when you try those samples in Windows 8 and change your language in the Control Panel, it does not work! The application always default to English! Why?
Because there is one crucial thing missing:
You need to set the supported languages in the application manifest file.
Open the Package.apppxmanifest file with a XML editor. Look for these lines and add your supported languages:
<Resource Language="en-us" />
My guess is this should be done automatically on compilation. But currently that's not happening.
It is possible to auto generate these values by setting the language value to x-generate:
<Resource Language="x-generate" />
Thanks Tim Heuer for the tip.