The Celts are possibly descended from a certain "Gaedheal Glas, son of Agnoman, who was in turn supposedly descended from Magog son of Japheth.
From Gaedheal we get Gael, Gaul and Gaelic. The fact these people originated in the same region as the Cimmerians, in Latin "Cimbri" and in Greek "Kimmeroi," has led Biblicists to identify the Celts with the Cimmerians and ultimately with Gomer, one of the sons of Japheth.
< Latin "Cimbri,"
< Greek "Kimmeroi,"
< Assyrian "Gimirru," (meaning "go back and forth," "nomads")
In all likelihood, a false etymon.
Biblicists are fond of making such connections. The Cimmerians were an Iranian group. The term "don" was the word for "river" which is why in regions where the Cimmerians dominated there are so many rivers that have this "dn" as a root, the Don, the Donets, the Dniepr and the Danube, but this doesn't stop the biblicists from identifying the Danube (Donau in German) with the Israelite tribe of Dan.
Was it a written language?
Cymraeg was probably written in both Latin and ogham scripts before 1000 AD.
Who knows what scrypt the Cimmerians may have used around 700 BC?