Delimiting thousandths..converting 12345678 to 12,345,678 for example

[Mark E. Powell]

If there a built-in "thousandths delimit" function that takes a number and inserts the appropriate delimiter (e.g. commas for USA).  Alternately, does someone have an idea for a really fast and efficient custom function that does it?  I have maybe a hundred thousand numbers at a time to convert and cannot think of how to do it reliably in less than a dozen lines of code. 

Corollary to this question is, any way to read user's system preferences about which delimiter to use?

Thanks

Mark