Gaulish
Gaulish is the name given to the now-extinct Celtic language that was spoken in Gaul before the Romans, the Franks and the British Celts invaded. It is only known from a few quite small pieces of writing.
It was a Continental P-Celtic language with a fairly complicated inflecting morphology. It had six or seven cases. Unfortunately for the future of the language, it was quite similar to Latin, enough so that when Cicero's brother Quintus was besieged by the Nervii in Gaul, Julius Caesar sent him a secret message in Greek, not Latin, so it could not be read by the enemy if they intercepted it. This meant that the Gauls had little trouble learning Latin, and quickly adopted it. The Gaulish language survived up to the time of the Frankish conquest in some areas. Gregory of Tours wrote in the 6th century that some people in his area could still speak Gaulish. However, the Gaulish language must have died out soon afterwards.
Enough inscriptions and texts in the Gaulish language survive to let us find that Gaulish was a P-Celtic language, having p where the other group of Celtic languages, Q-Celtic, have kw or k. Thus the Gaulish word for "son" was *mabos or *mapos, where Q-Celtic would have had maccos or maqqos, forms which are attested in Ogham inscriptions. Some of the Celts in Iberia may have spoken Q-Celtic languages; their language has left even fewer traces.
The longest surviving connected text in Gaulish is the Coligny calendar, found in Coligny near Lyons, France, which recorded the months in use in Gaul at the time of its use, and marks each day as lucky or unlucky. Gaulish was written in various alphabets that were brought to Gaul by outsiders; the Greek alphabet was sometimes used, as were the Latin alphabet and the Etruscan alphabet. The Ogham script, found in the British Isles, was not used in Gaul.
Breton is a Celtic language still spoken in Brittany. It is derived largely from the language of post-Roman British refugees fleeing from the Anglo-Saxon invasion and takeover. Breton has some features from Gaulish, but merely a trace.