This article concerns content exclusive to Warcraft RPG and thus unlikely to be canon.
Terran is a language spoken by earth elementals hailing from Deepholm. It is also known by conglomerates with the sub-type of earth. It is perhaps a dialect of Kalimag.
The term "Terran" shows up in the Kalimag phrase, "Ter'ran" in the Kalimag language parser.
This article or section includes speculation, observations or opinions possibly supported by lore or by Blizzard officials.*
"Terran" has an apparent connection to DnD derived generic languages which apparently can be found in DnD rule books.