Her star vehicle this fall is an early 19th-century opera, _Il Turco In Italia_ (The Turk in Italy), written by Gioacchino Rossini prior to his masterpiece, _The Barber of Seville_. _Il Turco_, presented by the New York City Opera for eight performances in September through November, is a subtle comedy about a flirtatious, Sophia Loren-type character (Sills) with a jealous husband. The audience will miss none of the Italian humor because this production of _Il Turco_ is in English. "I love to do English translations," said Miss Sills last week in a telephone interview. "I believe the whole art of opera is based on communication. I don't see how people can appreciate a comedy in a language that four fifths of the audience doesn't understand. There's only snobbery about foreign languages in this country -- not in Europe. In America, an opera is like a museum piece. But I think the great classics like _Boheme_ and _Traviata_ don't need to be translated because everyone knows what they're about." She performs regularly with the New York City Opera even though the State Theatre-based company is able to pay only a tiny fraction of what singers receive at other great opera houses around the world. "I made my career with them," she explained. "I sing there because of loyalty, and because I love to." She has already made plans to retire from singing in 1980 and to become codirector of the New York City Opera with Julius Rudel, the present director. Right now she is busy studying three other roles. On December 7 she will headline the Metropolitan Opera's new production of Donizetti's _Don Pasquale_, which will run until January 20. In March she will star in a world premiere for the New York City Opera, _Miss Haversham's Fire_, based on the Charles Dickens novel _Great Expectations_. In June she will go to San Diego to perform in yet another world-premiere opera, _Juana La Loca_ by Gian Carlo Menotti. Last season, Beverly hosted a popular television program called _Lifestyles_. This year, she said, "I'm doing something much bigger, as a result of that show's success. Unfortunately, I can't tell you what it is, because CBS will be making an announcement in mid-October." Miss Sills said she has no plans for another book. Her first, the self portrait _Bubbles_, has sold 130,000 copies in hardcover and many times that figure in paperback since it came out a year ago. "Bubbles" was her childhood nickname. She was born Belle Silverman in Brooklyn a few months before the stock market crash of 1929. At 3 she did her first radio broadcast; at 7 she was the star of a regular weekly radio show. In her early teens she joined a touring musical company and spent the next 10 years on the road. Then she was accepted by the New York City Opera.