Slightly literary / classical-flavored. Modern Chinese prefers
各国 (gèguó, 'each country') for 'all countries';
万国 survives mainly in fixed names —
万国邮政联盟 (Universal Postal Union),
万国博览会 (
世博 / World Expo, older name) — and in the famous
上海外滩万国建筑群 'Bund's collection of architecture from all nations'.