UN is good acronym for this because United Nations will become obsolete and spare UN acronym for better use.
United Nations is not accurate name for that organization. Better name is United Governments or United Powers especially that most governments no longer represent Nations.

Word Nation is both discriminatory and obsolete because it's quite hard to determine nationality of a child from 2 different nationals, speaking fluently 2 languages, citizen of 3 countries, lived equal % of life in 4 countries, does not have any plans to stay in one chosen country and has properties and ties with 5 countries.

Same as words race and ethnicity.
If someone needs good identification for security purposes they can use HTML style color closest to skin color of that individual in no-tan condition. They can even put it into passport and treat those with skin color darker than #3A3A3A or lighter than #F7F7F7 as protected minority for EOE employment purposes.