Click the 3-bar icon at the upper right, Add-ons. Which dictionaries are installed under Dictionaries? If the Italian dictionary isn't installed, type italian in the search box and in the next tab install Dizionario italiano.

In the message composer, the spellcheck dictionary can be selected at the right end of the Status Bar, at the bottom of the window.

The Dizionario italiano is installed properly and was selected for the email being composed. The email that was composed was from an old message which I used as a template by selecting "edit as new message". I don't know if perhaps this is why it didn't work. I have copied and pasted the content of the email into a new email message and the Dizionario italiano seems to be working now. I do not know why it didn't work in the first email, which was composed by selecting "edit as new message".

I don't know either, as Edit as New Message applied to a sent or template message here doesn't affect the dictionaries available in the Write window. Tested with the Italian dictionary. Maybe you have some other add-on that is incompatible, although I doubt it.