She sang, acted, and wrote, to great acclaim--but she was born poor and was only fourteen when she became a professional. How and where did she acquire the wit and knowledge to achieve what she did? Would she have had a private (even secret) tutor or teacher, like many professional singers of the day? Or was there perhaps a convent involved?
There is very little on her early life that I know of. However, the Republic of Venice paid for free primary education in all cities that surrendered to the Republic in the first decade of the fifteenth century. These cities were Verona, Vicenza, and Isabella's hometown of Padua. Although education certainly favored boys, there weren't institutional hurdles stopping a girl from acquiring an education, just social ones (although those can be just as cumbersome). Also, acting was a fairly approachable trade for a woman of her social station; she wasn't writing ecclesiastical texts or legal treaties, after all.
Female actors weren't unheard of, and she seems to have had a natural talent for improvisation, giving a new take on traditional Commedia Dell'Arte female stock characters. She also performed with a very talented company, of which her husband was a member; thus a fair amount of credit should also be given to the people she worked with who clearly supported her own natural talent. There wouldn't be anything scandalous about her ability to act, write, and sing; and her other talents, like languages and wit, could have easily been learned on the job given her natural intelligence. Plus, there would have been nothing scandalous about hiring tutors to help. Although I'm not exactly an expert on sixteenth century acting troupe employment practices, if her troupe was traveling to France or England I'm sure they might hire a tutor or guide to get them acquainted with the language, what she did with what she knew was up to her.