What is the recommend way to create a custom culture and associated resource files for a specific Client?

I have client that wants to specifiy their own version of localized content for a subset of my string resources.

For simplicity here is basic example:
Lets say I have 2 localized strings (showing english content)
PageTitle="Hello World"
PageDescription="This is a more wordy version of Hello World!"

I wish to localize these so I have resource files.

• Strings.resx (contains my English string)
• Strings.fr-ca.resx (contains my French-Canadian strings)
• Strings.fr-ca-clientX.resx (contains my strings for a Client whom is French-Canadian and therfore very picky;) - just joking)

Ideally "Strings.fr-ca-clientX" can specify only the strings they want to "override". In other words they may just wish to change the PageTitle and continue using the PageDescription from the "fr-ca" resource file.

So how do I go about this in .NET? Ideally I would just create the resx file and specify the culture in my "Web.config" and it should work...

<globalization uiCulture="fr-ca-clientX" culture="fr-ca-clientX" />

However, this does not work. "The tag contains an invalid value for the 'culture' attribute" is my first obsticle.

Thanks,
Justin