There will always be a conflict from the Catholic Church with nationalists, just like there are with anyone who promotes some other sin or heresy. Nationalism is diametrically opposed to the universality of the Church. It is an impossible fit within the confines of the teaching of Jesus and the Golden Rule. One can not treat others as they want to be treated while seeing themselves as deserving of exceptionalism.
Both of these statements are patently untrue. The issue is treated with some detail in CCC 1880 - 1927 and beyond. The more crucial teachings are:
1891 The human person needs life in society in order to develop in accordance with his nature. Certain societies, such as the family
and the state, correspond more directly to the nature of man.
1894 In accordance with the principle of subsidiarity, neither the state nor the larger society should substitute itself for the initiative and responsibility of individuals and intermediary bodies.
Note: This rules out both collective socialism and globalism when either one seeks to undermine the individual responsibility that underpins a truly free society, and that would seem their express reason for existing.
1897 Human society can be neither well-ordered nor prosperous unless it has some people invested with legitimate authority to preserve its institutions and to devote themselves as far as necessary to work and care for the good of all.
1901 If authority belongs to the order established by God, "the choice of the political regime and the appointment of rulers are left to the
free decision of the citizens. The diversity of political regimes is morally acceptable, provided they serve the legitimate good of the communities that adopt them. Regimes whose nature is contrary to the natural law, to the public order, and to the fundamental rights of persons cannot achieve the common good of the nations on which they have been imposed.
1911 Human interdependence is increasing and gradually spreading throughout the world. The unity of the human family, embracing people who enjoy equal natural dignity, implies a universal common good. This good calls for an organization
of the community of nations able to "provide for the different needs of men; this will involve the sphere of social life to which belong questions of food, hygiene, education,…and certain situations arising here and there…