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.