"Shall I legalize the Italian police certificate from PREFETURA, after I already had legalized it in tribunale?"
Those processes don't translate to anything I recognise.
You should do what it says here:
https://www.canada.ca/en/immigration-refugees-citizenship/services/application/medical-police/police-certificates/how/italy.html
On the assumption that you are an Italian citizen living outside Italy:
If you live outside of Italy:
You should apply in person or by mail for a
Certificate of outstanding criminal proceedings (
Certificato dei Carichi Pendenti) at your nearest Office of the Prosecutor of the Republic (
Procura della Repubblica). You do not need to go to the office closest to where you were born or where you live, however your certificate will only cover criminal proceedings and appeals at that office.
You should apply in person or by mail for a
Certificate from the Judicial Records Register (
Certificato del casellario giudiziale – General Certificate) at your nearest Judicial Records Office (
Ufficio del Casellario Giudiziale), which are attached to every Office of the Prosecutor of the Republic (
Procura della Repubblica). Only General Certificates are accepted (Criminal Certificates and Civil certificates are
not accepted). You must bring your valid identity document. If you apply by mail, you must send a copy of your valid identity document with your application.
If you cannot apply in person for either or both certificates, you may authorize someone to apply on your behalf. Your representative must bring his/her valid identity document.
"Shall I translate it to English in Italy, or at certified translator office in Canada?"
It doesn't matter