You can also hear many operas sung in German, French, Russian and even English 🙂 as well as Italian and others... Traditional Chinese opera can also trace it's history back well over 1000 years. Professional opera singers often learn multiple languages, to widen the scope of their careers.

I can’t understand a single word of an opera song even when it’s in English.I wouldn’t even be able to know it was in English.

Opera is sung in many languages, but particularly in Italian, Russian, German, French, and English. There is often a written Libretto that accompanies the opera so that people can understand the words. Opera contains stories, but you can learn the story from somewhere else, internet etc, and just enjoy the sound. Chinese opera also has stories, but it's quite different in style and sound because it is very old.

It’s old Italian. The italian language changed A LOT during the years (mostly from 1300 to 1900) and the italian lamguage as we speak it today was born only in 1861, with the unification of the country. It’s really hardto understand opera also for a native italian speaker. But at least now you know why 🙂