I have the same issue in our SharePoint projects, but gave up on it because it appeared that SharePoint wasn't applying the resource fallback mechanism like I expected it to.  Then I saw this post and decided to research this a little. 

I came across this article http://msdn.microsoft.com/en-us/library/ee696750.aspx and it says that a Default resource is:

> Also known as a fallback resource, default resource files contain
> strings localized for the default culture, such as English. They are
> used if no localized resource files for the specified language can be
> found. Default resources do not have separate files, they are stored
> in the main application assembly.

I didn't know this, especially that the default resources are embedded into the assembly. The instructions to add this are unclear:

> To specify default resource files in Visual Studio SharePoint
> projects, select Invariant Language (Invariant Country) in the culture
> list of the Add Resource dialog box when you add a resource file.

Turns out this is in the Properties of the Visual Studio project. Click on the Resources tab, and you'll see a message that the project doesn't contain any default resource files. 

However, I don't see the ability to choose the culture of the default resource file. I'll keep looking and update my answer if I figure it out.