Mozart first started to compose music when he was about 4-5 years old. His father worte them down for him. By 6 he had written a full on concerto, at 9, his first symphony, at 12, a full scale opera, Named Bastien und Bastienne. It was about the love of the tow lovers, Bastein and Bastinenne. And not to forget, he wrote this at the age of 12.
Opera began in Italian, and to compose in Italian was still the accepted tradition in the baroque era when Mozart was composing. This tradition held all the way through the classical period and onwards. While a few operas were composed in other languages by Mozart and others, Italian was the primary choice.