Complex sentence: One of the duchess's most accomplished works was a 166-piece collection of songs and piano pieces, copied by herself from 1811 to 1823 into five different languages and includes works from composers such as Rousseau, Gay, Mozart, Arnold, Arne and Handel.
Split into simpler sentences: One of the duchess's most accomplished works was a 166-piece collection of songs and piano pieces that she translated into five languages between 1811 and 1823. It includes works from composers such as Rousseau, Gay, Mozart, Arnold, Arne and Handel.