With all due respect to the authors of The Sundered Eagle, they did a terrible job with languages - which, arguably, are a very minor point of an otherwise superb supplement, and are hard to model precisely with Ars Magica mechanics. But it could have been done waaay better. I think this would be a better breakup of the languages in 1220 in Italy (listing in parentheses only the major dialects - in fact there are literally dozens if not hundreds of different dialects):
All derived from Latin and at -3 from each other unless otherwise noted:
In the north: Piedmontese, Lombard, Genoese, Emiliano-Romagnolo, Veneto (Istriot, Dalmatian), Ladin, all at -2 from each other and at -2 from Occitan, and with Veneto at -2 from Sabir.
In the center-south: Tuscan (1220 Corsican), Neapolitan (Abruzzese, Campano, Calabrese, Pugliese), Sicilian; with Neapolitan at -2 from Tuscan and Sicilian, and Tuscan at -2 from Emiliano-Romagnolo.
In Sardinia ... Sardinian.
See below for a more detailed analysis.
Note there exists no Italian in 1220. A person from Sicily (speaking some dialect of Sicilian), one from Tuscany (speaking some dialect of Tuscan) and one from Istria (speaking Istriot) will have at least the same trouble understanding each other as they would have understanding someone speaking Occitan. In fact, even today someone speaking Italian (strongly related to 1220's Tuscan) will have a very hard time understanding someone who speaks pure, uncorrupted Veneto or Sicilian - no easier than understanding someone speaking Spanish or Catalan.
A more detailed analysis.
You want to partition Italy very roughly as follows. Remember, Italy is like a boot, with a northern area immediately south of the Alps, and then a peninsula with the Adriatic sea on the west and the Tirrenian on the east.
a) The northwest, except for the coastline of the Tirrenian sea, let's call it Piedmont.
b) The thin coastline north of the Tirrenian, let's call it Liguria or the Genoese coastline (Genoa is the main city).
c) Central north, Lombardy, north of the Po river. Milan is its largest city.
e) The northeast, north of the Po river, including the northern coastline of the Adriatic. Let's call it Veneto, but excluding the mountains and hills to the north.
f) The northeastern Alps and their foothills, let's call it Trentino.
g) The eastern coast of the Adriatic, let's call it Dalmatia, all the way to (but excluding) modern Albania.
h) The eastern coast of the Tirrenian north of Rome, let's call it Tuscany.
i) The western coast of the Adriatic north of Rome, let's call it Emilia Romagna (in fact, Emilia Romagna is just a portion of that, but we need a single name). h) and g) abutt each other somewhere in the middle, of the peninsula, along the mountain ridge knows as the Appennini.
j) The eastern coast of the Tirrenian south of Rome (but north of "tip of the boot"), let's call it Campania (largest city is Naples).
k) The western coast of the Adriatic south(-east) of Rome (but north of the "heel of the boot"), let's call it Abruzzo. Again, the Appennini separate h) and i).
l) The "tip of the boot", Calabria.
m) The "heel of the boot", Puglia (Apulia, in latin).
n) Sicily, the big island that Italy seems to be kicking.
o) Sardinia, the larger and southern of the two big islands in the middle of the Tirrenian.
p) Corsica, the smaller and northern of the two big islands in the middle of the Tirrenian.
Now, as far as languages go! People in these areas all speak languages derived from Latin, except for some minorities (particularly a lot of slavic folks in g), some germanic people in f), and some pre-roman folks lost in the middle of m) and n) that I'd probably treat as faeries).
Piedmont (a) speaks Piedmontese, Liguria (b) speaks Genoese (or "Ligurian", but people would think of it as Genoese in 1220) Lombardy (c) speaks Lombard. These languages are fairly related, though probably not related enough to be dialects of the same language in Ars mechanics. Penaltywise, they are somewhere between -1 and -2, though I'd make them -2 if I had to choose. They are also fairly related to Occitan (about -2), and in fact they are called Gallo-Italic languages because they straddle Italy and Gaul (modern day France/Switzerland/Cataluna). I'd also include among these the language "Emiliano-Romagnolo" spoken in Emilia Romagna (i), some people leave it out and some even leave b) out; but it's a little easier this way.
Veneto (e) and Dalmatia (g) speak various dialects of Veneto; I'd consider Venitian one of them, Istriot another, and Dalmatian yet another. Making them dialects of the same language may be straining things a bit as they fall somewhere between -1 and -2 from each other (there are in fact dozens of subdialects) but I'd say it's reasonable. As mentioned above Slavic people from g) also speak their own, totally different Slavic languages. Veneto is fairly intelligible to speakers of Occitan (-2), and it's one of the main components of Sabir, the trade language of the Mediterranean, so it should be at -2 from that, too.
Trentino (f) speaks several dialects of a Rhaetian language which we can call Ladin (pronounced Ladeen) probably at -2 from Veneto, Occitan and the northern languages of the peninsula, and probably at -2 to -3 from the rest of the peninsula. Lots of areas here instead speak germanic languages, totally unintelligible with Ladin.
Tuscany (h) speaks Tuscan. That's somewhat related to Emiliano-Romagnolo (i), somewhere between -1 and -2, and far less related to Gallo-Italic languages and Veneto (somewhere between -2 and -3). Corsica (p) in 1220 probably speaks a dialect of Tuscan (basically because there are a bunch of former Tuscans there!) save for some shepherds in the hills who've been there since before the Romans. In fact, there are a bunch of dialects related Tuscan, all the way down to Rome, where they slowly start to morph into dialects related to Neapolitan.
Campania (j), Abruzzo (k), Calabria (l) and Puglia (m) speak various dialects of the Neapolitan family. Again, they are sometimes fairly "apart" (somewhere between -1 and -2), but for simplicity, I'll treat them all as dialects of Neapolitan.
If you want to be really precise, there's a fifth dialect of Neapolitan, called Lucan, in Lucania, the tiny area between the "tip" and "heel" of the boot. There are two exceptions: the very very extremity of the extremity of Calabria and of Puglia speak dialects of Sicilian rather than of Neapolitan (see below). These are somewhere beween -2 and -3 from every other language of the peninsula.
Sicily (o) speaks Sicilian, two separate dialects of which are spoken at the very extreme tip of Calabria (l) and Puglia (m).
Probably at -2 from Neapolitan, and at -2 to -3 from every other language of the peninsula.
Sardinia (m) speaks Sardinian, which is its own language, still derived from Latin but probably at least at -3 from every other language of the peninsula.
