public static void addResourceBundle(String baseName, ResourceBundle resourceBundle) {
        Objects.requireNonNull(baseName);
        Objects.requireNonNull(resourceBundle);
        Locale locale = resourceBundle.getLocale();
        if (locale == null) {
            bundles.put(baseName + "::en", resourceBundle);
        } else {
            bundles.put(baseName + "::" + locale, resourceBundle);
        }
    }
Gets the locale to use when finding a bundle.  The locale to use is either from the
thread local value, if set, or else the system default locale.
@return the locale
common/util/src/main/java/io/apiman/common/util/AbstractMessages.java