In Common Lisp, such directives use : rather than ', but more importantly they also take arguments to specify the grouping and the separation character. They default to 3 and comma respectively. If Emacs' format were to allow similar arguments to be provided, it would allow the exact format to be specified where it is important and the user could set global variables like default-digit-grouping and default-digit-separator or some such to get behavior matching their locale. For example:

(format "%:d" 100000) => "100,000"

(let ((default-digit-grouping 4)
(default-digit-separator ?.))
(format "%:d" 100000)) => "10.0000"

-- Graham Dobbins