Before I get to any type of written language, I must first say that it depends on what you mean by 'Celt' and how far back in 'Celtic' history you are thinking. Although I am no expert, here is a bit of what I have gathered through research over the past year, although I have been concentrating on Celtic Art.
It is my understanding that when most people think of 'Celts' they think of the Irish and Scottish people, and most certainly the modern-day Irish and Scottish think of themselves as Celtic. Although over the last 2500 years or so many people have written about the Celts, but there is absolutely no primary written documentation of anyone referring to the native British and Irish people of the Iron Age as Celts, and the native British and Irish people of the Iron Age certainly did not refer to themselves as Celts either.
"According to the world view of Greek historian, Ephorus of Cymae, in his Universal History from the first half of the fourth century BC it is stated, "`If the heavens and earth are divided into four parts, the Indians will occupy the land of the east wind, the Ethiopians the regions from which the south wind blows, the Celts the west and the Scythians the land of the north wind.'" This quote presents the first steps to understanding the Celts; that is they covered a vast area of Europe thousands of years ago and they were not yet on the British Isles." That was a paragraph from one of my essays.
Some of the most notable primary sources on the Celts are also by Julius Caesar and Tacitus.
My guesses are that some Celtic traditions were spread, if not some Celtic tribes themselves, during the Migration Period of various 'barbarians', which the Celts were considered as. Also, it is believed that native British people had migrants from Gaul (modern-day France) because there is an over-lap of Druidism between the two areas.
As for written languages among the Celts up until the 3rd century CE there does not appear to be any at all. Frankly, the Celts would not have been concerned with recording anything in writing because they were a people who put strong value in war, metal engravings, and oral tradition. However, and I really don't know for sure if they would have been considered 'Celts' (but I doubt it), during the 3rd century CE or so the Irish developed Ogham script, which you should Google it if you wish to see what it looks like. However, this script is predominately concerned with genealogies and not recording stories or history. So Irish Ogham is the best I can give you. If you are interested in the Celts check out Barry Cunliffe's The Celts A Very Short Introduction.
The Celts primarily spoke Celtic languages, such as Gaulish, Brythonic, and Goidelic. The Romans spoke Latin.
The Celts influenced the English language through the introduction of Celtic words into Old English, the language spoken in England from the 5th to the 11th centuries. These Celtic words contributed to the development of the English language and can still be found in modern English vocabulary.
The language in which the document was first written is not specified in the question.
The word "Celts" in Irish is pronounced as "keltz." The "c" in "Celts" is pronounced as a hard "k" sound.
written language developed by the Aryans
AFRO CELTS. has written: 'SEED'
John Patterson MacLean has written: 'On the Kelto-Saxon' -- subject(s): Celts, English language
Of or pertaining to the Celts; as, Celtic people, tribes, literature, tongue., The language of the Celts.
The Celtic Amazigh of North Africa, unlike the later European style 'Germanic Celts' that Eurocentic historians insist upon , did have a language of symbols, a full alphabet one of which was the sign we know as the swastika. In their long dispersal then gradual absorption and elimination, the Celts of Europe lost their written language however and became illiterate. The Amazigh of North Africa did not lose their written language and continue to this day as an independent people albeit under the name given to them by the Romans of 'Berbers' ( barbarians).
Gaelic
The Celts primarily spoke Celtic languages, such as Gaulish, Brythonic, and Goidelic. The Romans spoke Latin.
i think it was gaelic. i'm not quite sure
The Celts influenced the English language through the introduction of Celtic words into Old English, the language spoken in England from the 5th to the 11th centuries. These Celtic words contributed to the development of the English language and can still be found in modern English vocabulary.
Klaus Holitzka has written: 'Mandalas of the Celts' -- subject(s): Celts, Coloring books, Mandala, Religion 'Mandalas celtes'
English. Look up the Anglo-Saxons and the Celts.
Martha Sills-Fuchs has written: 'Wiederkehr der Kelten' -- subject(s): Celts 'Knowing wisdom' -- subject(s): Knowledge and learning, Celts
The English language came from German. Read up on the Anglo-Saxons invading the Celts. England used to belong to the Celts (Irish/Scottish) but then the Anglo-Saxons (the Germans) invaded. (This was before the English language.) They called their new land Angleland. Later, after they created the English language, they re-named it England.