Bandello's work, first published in Italian in 1554, was popularly translated into French by Francois de Belleforest as "Les Histoires Tragiques," in 1580, and it is quite certain that Shakespeare knew this book and consulted it. He may also have read enough Italian to make out the sense of the tales in the original. The original stories of "Twelfth Night" (Novelle 36); "Romeo and Juliet" (Novelle 9); "Much Adoe about Nothing" (Novelle 22) are all found in Bandello.
Ref.: Catalogue of the Exhibition of Shakespeareans Held at the New York Public Library, April 2 to July 15, 1916, in Commemoration of the Tercentenary of Shakespeare's Death by Henrietta C. Bartlett, New York, 1917 (p.77).
Note: Six of Bandello's stories was first translated into French in 1559 by Pierre Boaistuau. Francois de Belleforest translated twelve of Bandello's stories in 1570.