Locale override when installing XPIs

Hey all,

If I'm trying to do the right thing and localise my XPI using a 
'locale/en-US' directory, what's the best way to make that 
fault-tolerant?  ie. if the person who downloads my XPI is from Great 
Britain, not the U.S., how do I create an XPI that will also work for 
their locale?

Is there something clever I can do with an install.js script?

Ideally, the process would work something like this:
(1) User clicks on XPI install link
(2) install.js checks current locale against locales available with this 
package;
(3) if found, continue on, else prompt 'This package does not contain 
the right locale information' -> user selects locale to use, or cancels out.
(4) If user selects locale, a locale directory is created in Mozilla 
*under the correct locale code* for that user, but filled with a 
'default' set of values.

But I don't know how to do locale comparisons.  Any clues?

Thanks,

GuruJ.