One overriding note is that even when a nation or province speaks one language more or less uniformly, it is still possible to travel ten miles and encounter mutually incomprehensible versions of the same language. A modern example of this is Swiss German, Bavarian German, and modern High German. Exactly where the linguistic line is drawn between having different dialects or different languages is well beyond the scope of this meager listing. Use your good judgement.
[N.B. The languages begin to diverge in the C. 10th because of geographical separation. Irish is originally written in the Ogam (or Ogham) alphabet. In this period, Scots Gaelic is written in the Irish dialect.]
[N.B. Cornish and Breton are mutually intelligible until the C. 15th. Welsh in 1200 A.D. is Old Welsh. In 1200 A.D., Cumbrian is probably similar to northern Welsh. Literature in these languages is written in the period from the C. 12th to the C. 15th.]
[N.B. The various Scandinavians languages became distinct starting in the 12th century. Old Norse is still the language used for the Icelandic sagas. You may consider the Scandinavian languages presented to all be dialects of Norse.
Dutch includes Flemish dialects in Belgium. I do not know how widespread Yiddish might be in Mythic Europe given how Jews are treated by society at large, but it is probably prevalent in mostly Jewish villages. High German dialects include Franconian, Swabian, Swiss, and Rhenish.
Old English (Anglo-Saxon) and Frisian are still very closely related. English has probably begun to absorb some elements of French in the 150 years since the Norman invasion, but this French influence is most prevalent in the southeast. Also, most English nobles at this time speak French in court. Major English dialects include Mercian, Middle English, and Northumbrian. ]
[N.B. Some other contemporary Latinate languages from the north of Italy include Faliscan, Oscan, Umbrian, and Venetic, but these are mostly dead by 1200. Major Italian dialects are regionally based, and include Corsican, Lombard, Neapolitan, Roman, Sardinian, Sicilian, and Tuscan. Rhaetian, which includes several (mutually incomprehensible) languages, is spoken in the Piedmont region.]
[The major dialects of Langue d'Oil are Normand (Norman French), Picard, and French. The major dialects of Langue d'Oc are Catalan and Provençal. The dividing line is roughly the Loire River.]
[Castillian and Portuguese are very similar languages.]
[N.B. Most South and East Slavonic languages use the Cyrillic alphabet. Croatian uses the Latin alphabet although the spoken language is very close to Serbian.]
References include correspondence on the Ars Magica mailing list (the Berklist), especially Jakob Ryngen, Michael DeVerteuil, and Patrick Juola. The main source is the Cambridge Encyclopedia of Language.