IIRC, Hoshi was teaching at a language school in Brazil during the Enterprise pilot, with the country being mentioned by name.
Also during Enterprise, Reed stated that Britain stiil retain a naval force, suggesting that it was still a sovereign entity.
Per Beverly, (in a hypothetical) Australia still existed in the year 2161. She didn't seem to be referring to the continent, but to the country.
Picard said that Riker (who was from Alaska) and George Armstrong Custer (who was from Ohio) were "countrymen." This suggests that the United States, in some form, remains a nation in the 24th century.