A critique of Stalin's notion of Nationalism: (thread)
Lenin and particularly Stalin outlined what constitutes a nation through economic relations, culture, territory, and language. On the surface, this analysis has much merit, and it does have good points, but still misses...

an aspect of nationalism. This analysis suffers from assuming that the nation isn't a social construct which is libel to have arbitrary characteristics. In practice, Stalin's definition has also more often failed than worked out, especially outside of Europe.

In Europe, these terms make some sense, but outside of Europe, in Africa, the Americas, and the Near East, this definition begins to lose merit. Take Syria, a product of colonialism and now an opponent of imperialism yet Syria should have no right to exist according to Stalin.

Neither should any country that has significant ethnic minority groups. In theory, they likely should all be separate nations, yet they all identify with a nation that's very conception is a hold-over from colonialism. This issue is written off in the name of anti-imperialism...

but this is unsatisfactory. I could get into how Stalin himself ignored his own definition, but we'd be here all day.
In my opinion, our support for a national movement should hinge on 2 items: strategic advantage for our aims, and feasibility.

The HK independence movement doesn't serve our aims, so we oppose it. Catalonia's does so we support it.
And finally, there has to be a genuine mass appeal for the national identity prior to our intervention. No Marxist movement has birthed a new national identity for a reason.