Sumerian language. Sumerian language, language isolate and the oldest written language in existence. First attested about 3100 bc in southern Mesopotamia, it flourished during the 3rd millennium bc. About 2000 bc, Sumerian was replaced as a spoken language by Semitic Akkadian (Assyro-Babylonian) but continued in written usage almost to the end of the life of the Akkadian language, around the beginning of the Christian era.