Fixed so that explicitly giving it an unknown language returns #f
instead of the first translation:

(define (make-translated-error . translations)
   (let ((languages (plist-keys translations))
         (first-one (cadr translations)))
     (make-foreign-error
      'languages
      languages
      'message
      (case-lambda
        (()
         first-one)
        ((language)
         (if (not language) first-one
             (plist-get/default translations language #f)))))))