Ok, I found the missing method:

        Optional<UserRepresentation> user = foundUsers.stream().findFirst();
        if (user.isPresent()) {
            Map<String, List<String>> attribs = user.get().getAttributes();
            if (attribs.containsKey("language")) {
                attribs.get("language").set(0,"hu");
                userResource.get(user.get().getId()).update(user.get());
            }
        }