[Language is an extremely slippery topic when one tries to be the least bit realistic. This essay tries to be useful without being over done. Especially when "You speak Anglo-Norman and Anglish" works as a simple default.]
Most people in the campaign, including all PCs, can speak Anglo-Norman, which serves as the Common Tongue for the campaign. Persons of low social standing will tend to have more Anglish grammar and vocabulary, while those of high rank or aspiring to such a position will tend to use more French.
Anglo-Norman is the native tongue of very few people, so almost everybody (including all PCs) speak a second language. For locals to the setting that means either Anglish, French, Cornish or Welsh. Choosing French as your native tongue probably indicates either that someone in your family (father, uncle, older brother or half-brother) is a knight (which at this point in history is not yet a hereditary title) or that you are from France. Some folks originating from just off to the northeast of the map may actually speak a local dialect of Danish, as those ratfink Danes controlled a vast chunk of eastern England not that long ago.
Of course, you can play a foreigner type and speak some other tongue. The Gaelic tongue of the Irish, Scotts and Manx have yet to substantially diverge, so all those guys effectively speak the same thing. The aforementioned Danish will do you for all your stock viking types as well. Other very interesting options would be Hebrew for a Jewish character or Arabic for a Spanish Moor. Flemish mercenaries figure in some English armies of the period. Such characters would probably speak Frisian or Dutch or maybe German or French. The County of Flanders seems to be a pretty multilingual sort of place.
Latin serves as the Common Tongue for western Christendom, though almost no members of the lower classes speak it. All magic-users automatically speak Latin and it assumed that their spellbook is written in this tongue unless they are a native Hebrew, Arabic or Greek speaker. Any PC who is not a magic-user may start play knowing Latin if their Intelligence score is 11 or higher.
For simplicity’s sake all PCs are assumed to be able to read, with a low Intelligence score indicating various difficulties such as needing to form the words aloud, slow reading speed, limited vocabulary and/or poor comprehension. Writing is considered a separate skill and among PCs only magic-users have the necessary training.
Many magic-users can read and write (though perhaps not speak) a bewildering array of ancient languages or obscure languages. For each point of Intelligence above 11, they may select fan additional language. Below is a d12 chart with some possibilities. Pick or roll.
1 - Latin
2 - Greek
3 - Hebrew
4 - Greek
5 - Aramaic
6 - Egyptian Hieroglyphics
7 - Norse Runes
8 - Basque
9 - Enochian
10 - Old Irish/Ogham
11 - Persian
12 - Atlantean