Translation:The country is Finland and the language is Finnish.
”Suomi on kaunis” = Finland is beautiful. The adjective kaunis is in the nominative. ”Suomi on kaunisTA” = Finnish is beautiful, the adjective kaunis is in the partitive when describing a language. You can add more context: ”Suomi on kaunis maa” = Finland is a beautiful country, or ”suomi on kaunis kieli” = Finnish is a beautiful language