Build world-ready Metro style apps using XAML

There were some questions about fallback among regional variants. E.g., the app has strings for en-US and de-DE with de-DE as the ultimate fallback; but the user's preference is for en-AU. In this case, the en-US strings will be used. The system will look at the user's preference and infer a fallback chain that allows for regional variants. So, in the above example, it will assume a fallback chain as follows:

• en-AU  (user's first choice, but not available in the app)
• en        (region-neutral asset -- not available in this app)
• en-*      (any other regional variant -- will pick up en-US in the app)
• de-DE   (app's ultimate fallback)

Note that, as explained by Tim Heuer, you don't have to declare the resources using a specific regional variant language tag. You can, instead, use a region-neutral tag like "en". And you can use any combination of specific-regional variants as well as a region-neutral variant.

How does the Uid property interact with primatives?. 

All of the samples define the application title as a String resource in either the page or the App. There's a Uid property available on it, but I'm if I try to set it, I get a runtime exception for a missing resource. 

Is there going to be some design time indication what is and isn't supported to avoid runtime issues later?