**French was the "[Lingua Franca][1]" 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 replaced Italian. **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**.


  [1]: https://en.wikipedia.org/wiki/Lingua_franca