Anglish is the most common language used in Highland. It is basically English. There is also Celtic spoken in the northwest among the Celts, and a Germanic language among the barbarian tribes in the northeast.
Scholars and sorcerors also use the Ancient tongue, which is a bastardized form of Latin. Because of the relative paucity of Latin texts that survived the Cataclysm, you should feel free to make up Ancient phrases without worrying too much about whether you get the Latin “correct”. There are several English-Latin dictionaries available via web searches.
Frankish speakers (basically, an old French) sometimes travel north from Great Bend to Crosspoint, and can be encountered there or rarely as part of merchant caravans crossing the leather road.
While it doesn’t appear to be related to the Ancient tongue, the language of the “traveling Romans of Great Bend” is called “Old Roman”.