The earliest extent translation of the Ramayana is the Tamil Kampan Ramayana from the late 12th century. The might have been a Khmer translation earlier. The Assamese Kotha Ramayana is about two hundred years later. The Jain Ardhamagghadi Ramayana is very old, perhaps older than the Valmikiramayana!

I stand corrected about Assamese's relationship with Bengali.