Kharsum, known informally as Orcish, is the most prevalent language used by orcs.
While most orcs speak Lower Miezan fluently, they generally learn it as a second language at the same time as their native language, which is Kharsum for most orc tribes. Lord Black mentions when he has Catherine learn it that it is not the only language of the orcs, just the most common.
Kharsum was the oldest written language to be used in Calernia. Its popularity dropped considerably when the Miezans conquered the orc tribes and presumably required them to speak their own language instead.