Singing in Italian

Opera was invented in Italy near the end of the 16th century to recreate the classic Greek drama experience, as it was believed that the parts of the Chorus were sung.

Indeed, most of the Greek tragedies that we know and love included a chorus, and some manuscripts include some form of musical notation. Unfortunately, the meaning of these notations are lost to history, and nobody knows how to interpret them. There's no Rosetta Stone for the music.

Since the Italians invented opera, it's not a big surprise that they were all, at least originally, written in Italian. However, as opera naturally evolved and changed over time, the language stopped being exclusively Italian. French composers started using French librettos, British composers started using English librettos, and German composers ... kept using Italian librettos.

Why did German composers like Mozart continue writing their operas in Italian?

The answer, oddly enough, is that Italian is easy to sing. Italian has no dipthongs in the language whatsoever, and the vowel set only uses one primary pronunciation. Note that in making this statement, I am using "proper" Italian as the basis; no doubt there are regional accents and exceptions, as there are to any linguistic rule one cares to name. With German on the other hand, dipthongs abound and there are numerous ways of interpreting the vowels with every sound from long to short to umlauts possible.

Mozart didn't exclusively write Italian operas of course, but the three he is most famous for, namely The Marriage of Figaro, Don Giovanni, and Cosi Fan Tutti all are.