Final answer:
The Indus Valley Civilization created the Indus script, an undeciphered written language associated with early urban settlements like Mohenjo-daro and Harappa.
Step-by-step explanation:
The early river civilization that created a written language which experts still have not been able to decipher is the Indus Valley Civilization. This civilization developed a complex writing system known as the Indus script, prominent in archaeological sites such as Mohenjo-daro and Harappa in present-day Pakistan and India. Despite numerous attempts, the script remains undeciphered, leaving many aspects of this civilization's culture and history a mystery.