We could have the API:

(foreign-error:message ferr [language [country]]) -> string or #f

That makes all the following work easily:

(foreign-error:message ferr)           ; any locale
(foreign-error:message ferr 'en)       ; any English
(foreign-error:message ferr 'en 'gb)   ; British English

The message procedure given to `make-foreign-error` would then take two
arguments which may be #f: (lambda (language country) ...)