Language is, naturally, the main method of communication between groups of people.
Out of Universe note: there are large differences between the languages of "The Once and Never Kings" and OTL. English as we know it doesn't exist, divided between the Danish-influenced East Anglian in Jorvik, and the still heavily Germanic Saxon, spoken in Wessex. Luxembourgish is also more widely spoken, almost on equal footing with Dutch. French and Italian are more divided into their respective dialects, thanks to the heavy division in the regions.
Germanic is the language family with the largest number of speakers in Europe. Initially in three groups, the East Germanic family became extinct with the death of the last Crimean Gothic speaker in the eighteenth century. Today, only the members of the West and North Germanic groups remain.
Second only to Germanic in Europe, the Romance languages are primarily clustered in south and west Europe. It also has the largest number of sub-languages. This fact means that effort needs to be made for two speakers of different sub-languages even in the same group (such as Norman and Occitan, or Lombard and Sicilian) in order to reach mutual intelligibility. The Romance language family is divided into three groups: Western Romance, South Romance, and Balkan Romance.
The Celtic language family is descendant from the languages of the Celts, a group of people who used to inhabit much of Europe, but receded to the Albionic Isles during Roman times. After the Anglo-Saxon migration to England, the Celtic languages receded further. Some Celtic languages have been suppressed in the past, like Cornish and Welsh in Wessex, and thus have diminished in some areas. On continental Europe, the only country with a Celtic language with official status is Brittany. Celtic languages are divided into two sub-groups: Brythonic and Goidelic.
Balto-Slavic refers to a family of languages stretching across much of Eastern Europe. Ethnically speaking, the Balts and Slavs are distinct groups, though their languages have some aspects that justify grouping the two families together. The Baltic language family is further divided into two groups: Western and Eastern Baltic, while Slavic is divided into three: Western, Eastern, and Southern.