The word 'internationalism' is correct and usable in written English.
It refers to a policy of cooperation among nations, or between people from different nations. For example, "The Prime Minister affirmed his commitment to internationalism in his recent speech about global trade."
Dictionary
internationalism
noun
Political, economic and cultural cooperation between nations
synonyms
Exact(60)
As an instrument of the internationalism it was set up to pursue, it is hopeless.
The conference agreed a policy of a social alternative to austerity and privatisation; a green sustainable economy; a modern republic for real democracy; and internationalism based on opposition to Nato and Trident.
The bigger picture was a strong, but practical internationalism centred on the European project.
Unless it does this, proletarian internationalism will remain a meaningless phrase; mutual confidence and class solidarity between the workers of the oppressing and oppressed nations will be impossible.
His core ideals – a belief in social justice and a committed, practical, internationalism – remain as relevant as ever.
It is all to do with the death knell of the reasonable internationalism of people like Ken Clarke.
Labour is the party of internationalism and openness.
But compared to Labour and the other political alternatives, the Greens are now the most progressive force in British politics, with our visionary agenda for grassroots democracy, social justice, human rights, global equity, environmental protection, peace and internationalism.
"I think we may be moving towards a new period of internationalism to tackle issues like the environment, security and tax jurisdiction, not thinking about splitting into smaller units".
Business education over the last 20 years has been more or less predicated on internationalism and diversity.
But as other schools cottoned on to the virtues of internationalism, it found its thunder stolen.
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Since I tried Ludwig back in 2017, I have been constantly using it in both editing and translation. Ever since, I suggest it to my translators at ProSciEditing.

Justyna Jupowicz-Kozak
CEO of Professional Science Editing for Scientists @ prosciediting.com