I’m not talking about vulgar Latin or the romance languages.
For about a millenia and a half, everything that could be considered scholarship was written in Latin. Newton’s Principia Mathematica? Latin. Copernicus’ De revolutionibus orbium coelestium? Latin. Kepler’s Astronomia nova? Latin.
Almost every educated person in the western world learned Latin. That’s how they communicated with their colleagues in other countries - letters written in Latin. That’s why it was a lingua franca.