I think it won't. I haven't tried to switch on subtitles but menus and texts remain in English.
Hi i'm playing the game in spanish, and all is is in spanish (text and voices).
if you have the original disk in Italian,
go to the "C: \ Program Files (x86) \ GOG Galaxy \ Games \ Mafia" folder.
1- Delete the folder "sounds"
2-Copy this files from the disc and replace them in the gog version of mafia
Pd:I did it that way and it worked perfectly.,hope this works for you.