In an answer of another question concerning the link between communism and nationalism, it was stated "communism was even seen as a way to achieve national liberation and self-determination".
How did that unfold precisely?
I mean by that, who were the political figures which used communism as as a way to achieve national liberation and self-determination, and how did they translate that in words (e.g. statements in speech, personal communication, monographs/articles) and actions?
Socialism becomes a way to achieve self determination when the class structures in a society fall along ethnic lines, with the indigenous people forming the proletariat.
In this context you should not be looking towards the Soviet Union, but further East, to Vietnam.
Throughout the 19th century Vietnam was under colonial dominion. Looking further back it has been in a Chinese sphere of influence, with various Chinese Emperors taking greater or less control of the region. And during the 1940s it was controlled (through a French puppet) by Japan.
So for Ho Chi Min, socialism was a force for anti-colonialism, and hence national self-determination. Since the colonising powers (France, Japan, the USA) were capitalist, and so a class struggle was a national struggle: the proletariat were Vietnamese, and the Bourgeoisie were foreign.
This is, of course, simplistic. When reading Marx, you should consider that he was writing in the context of nineteenth-century Europe. His notion of "workers of all nations, unite!" is coloured by his limited experience of nations outside of England, Germany and France.
To this end you should see the speech given by Ho Chi Min declaring independence. As you read this, remember to mentally translate "democratic" to "people-power" and so "communist". Democracy - in this context - doesn't mean "elections" but "government by the party of the people".
And the action was the Vietnam war(s), to unify the country under a communist party.
I don't think Marx himself would have seen "national liberation and self-determination" as an important issue, since he was concerned with more abstract philosophical conflicts surrounding private ownership of collective production. But it's worth bearing in mind that from the late 19th and through the 20th centuries most of the world (outside of Europe, Greater Britain, and the USA) was comprised of colonies, banana republics, puppet government and client states: e.g., governments that were imposed, funded, subsidized, or otherwise propped up by the powerful states of the Western world, often (if not usually) for the economic benefit of private actors in those states. We can think of it as the evolution of 18th century colonialism, where rather than sending Western troops and governors to administer resource extraction directly, local regimes were established, armed, and/or effectively bribed by larger powers, with the understanding that disobedience would entail the withdrawal of that support and subsequent collapse of the regime. This created the 'Che Guevara' type of communism, which explicitly aimed to overthrow these capitalism-dominated regimes and reach for (you guessed it) national liberation and self-determination.
Both are in their idealism and rhetoric liberation movements (when in the opposition). Nationalism is arguing for a united nation struggling for a country of their own to exercise their self-determination. While the communist narrative asks for a united working class to strive for a society of it's own to exercise their self-determination both economically and politically.
So if you follow pure pragmatism with the goal of rallying up and organizing people towards a revolution against an oppressor then you can just focus on "us vs them narratives" and "a necessity for unity and an organized rebellion" on the one hand and talks of communal spirit and political and economic freedom, prosperity and self-determination on the other hand.
Conveniently ignoring for the moment that the two ideologies use these ideas very differently and concrete practical proposals as to how to achieve these things may very drastically, same with questions regarding who's part of the in-group and who's part of the out-group. But when it comes to organizing a rebellion often times beggars and choosers.
Also worth nothing that this goes both ways. Like a nationalist movement can insert their quest for a nation of their own in the liberation struggle of a communist party (if that already has an organization, resources, maybe outside support and whatnot), as well as a communist party lending their support to a nationalist movement that is close to achieving a critical level of dissatisfaction with a system. They would hit the same beats and try to shift the melody towards their favorite lyrics.
And as far as I know that became the most relevant in former colonial systems. Often enough colonial powers worked with a divide and conquer strategy where they didn't take control themselves but rather supported a puppet regime that was strong enough to keep the managerial control, but weak enough to rely on the colonial power to back them up if needed and thus not to establish independence of their own. So there again was an overlap between a class and nationalist struggle, where the ruling class and their supporters could be seen as a convenient enemy to rally against that both reflects a nationalist as well as a class struggle.
However when the struggle had been successful, which it often was if they were able to rally large parts of the population in a collective existential struggle, things usually fell apart again. Sure a "socialist country" would still exemplify versions of nationalism if it persisted as an independent country and/or would be placed under a different puppet regime this time by a "communist" state that support the revolution, while if the nationalist side was stronger the communist preface of the revolution is likely dropped or made more symbolic than ideological.
Regarding their ideology the nationalism usually demands to put the nation first so that the problems of the working class come second to the problems of the nations who's course is usually decided by the powerful people in economics and politics. Hence nationalism is not really compatible with the communist ideal and Marx rater argued for members of the proletariat to unite regardless of nationality than to have fights of nation was nation that just give more motivation to uphold the inequality and kill workers to further make a revolution more difficult.
So communism (ideally) isn't very nationalist, while nationalism also puts it's priorities elsewhere, that is not in the individual workers but in a national goal (also likely not chosen by the individual worker).
When in comes to marxist-leninist states however that organized as countries, nationalism might have been instrumental to appease people with a national goal disregarding the individual worker.
Marx, whilst a theorist was also a committed activist. Martin Luthor King was so impressed by the dedication that communists had to social justice that he gave a speech saying so and cajoling his flock and Christendom to say that whilst he couldn't go along with their athiesm they had a lot to learn from them about sheer dedication.
Communism took down tsarist Russia with their Red Army pushing back hard against the Nazi's when they invaded Russia. It also helped Vietnam to fight off the fascism of the USA and liberated Cuba. And Cuba was there alongside Angola against the invasion of Zaire and South Africa when the USA supported the aparthied regime in South Africa. Socialism also had a play to part in the fight for freedom of the Palestinians when Israel, under the auspices of the British Empire, stole their land for themselves. It was part of the political ideology of the PLO - as well as nationalism. And they were the face of the freedom fighters for Palestine.
Sure socialism/communism they made many mistakes, for example Stalinism. But which movement or people hasn't? Take Capitalism for instance - there are the crimes of colonialism, the transatlantic slave trade, global warming ...
