default void warnOnce(UserDto user, NotificationDto<? extends IVersionedApimanEvent> notification) {
        String langTag = user.getLocale().toLanguageTag();
        String reason = notification.getReason();
        String key = reason + langTag;
        if (!WARN_ONCE_SET.contains(key)) {
            LOGGER.warn("No email notification template could be resolved for {0} with locale {1}. "
                                + "No email notification will be sent, and all future notifications of this same type will be dropped.", reason, langTag);
            WARN_ONCE_SET.add(key);
        }
    }
@author Marc Savy {@literal <marc@blackparrotlabs.io>}
manager/api/rest-impl/src/main/java/io/apiman/manager/api/notifications/email/handlers/INotificationHandler.java