French was the "Lingua Franca" for diplomats in Europe from mid 16th century to First World WarFrench was the "Lingua Franca" for diplomats in Europe from mid 16th century to First World War, when English replaced French. French was used by diplomats in Spain, Italy, Germany, etc also in Sweden and Russia.
French did replacereplaced Italian, which was the main cultural and also diplomatic language in Europe since the 1400'(this. Italian was the main cultural and also diplomatic language in Europe since the 1400'. This is, since the Renaissance).
Latin was an academic language well into the 19th century, but latin was not used primarily by diplomats, altought Latin is occasionally found in diplomatic correspondence.