ruso

Esperanto builds nationalities from the person, not the country, which is the reverse of English habit. Ruso is a Russian, rusa is Russian as an adjective, la rusa on its own is the language, and rusino is a Russian woman. The country is then derived: older texts write Rusujo with the container suffix, modern ones usually Rusio, and both are understood everywhere. Zamenhof was a subject of the Russian Empire and Russian was one of his home languages, so the early literature is thick with rusoj and with Russian idiom showing through the seams.