The indigenous languages of the British Isles are all members of the Insular Celtic branch of the Celtic language family.
The languages of the other major branch, the Continental Celtic languages, are all now extinct. Of these, Celtiberian was found in north-central Spain, around Saragossa (Zaragoza). Gaulish was spoken in France, Belgium, Luxembourg, northern Italy, most of Switzerland, Germany to the west of the Rhine, and parts of the Netherlands. And Galatian was spoken in central Anatolia, Turkey: it was probably these Celtic-speaking Galatians who St Paul was addressing in his “Epistle to the Galatians”, even if he did write it in Greek.
Of the modern Insular Celtic languages, Welsh and Cornish and its close relative Breton are classified by linguistic scientists as Brittonic (or British) languages: Breton is the result of trans-Channel colonisation from Devon and Cornwall. Irish, Scottish Gaelic and Manx are Goidelic (or Irish) Celtic languages: Manx and Scottish Gaelic stem from the colonisation of the Isle of Man and western Scotland from Ireland.
The Goidelic and Brittonic languages are sometimes also known respectively as Q-Celtic and P-Celtic, because of a change in pronunciation which took place in early Celtic, probably before 800BC, whereby kw- changed to p- in some Celtic-speaking areas but not others.
The change from kw- to p- is a rather common one in the histories of the world’s languages. Latin quattuor “four” has remained quattro in Italian and cuatro in Spanish but has become patru in Rumanian. Latin aqua “water” is similarly apă in Rumanian. Latin equus “horse” corresponds to Ancient Greek hippos.
If you compare what your lips are doing when you say quick and then pick, you can perhaps see why this change can happen. The pronunciation of kw- and p- both involve the lower and upper lips coming together.
In the Q-Celtic languages, kw- later changed to k-, so there are now many words where Irish k- corresponds to Welsh p-. Ancient Celtic kwetuores “four” is ceathair in modern Irish and pedwar in Welsh. Original Celtic kwenkwe is Irish cúig but Welsh pump and Cornish pymp. Irish ceann “head” is pen in Welsh. And the Cornish male personal name Piran corresponds to Irish Ciarán (often anglicised as Kieran).
There have also been consequences of this change for our surnames. Gaelic mac “son” corresponds to Old Welsh map (Modern Welsh mab). Mac has been involved in the formation of scores of originally Gaelic surnames which are now very common in the English-speaking world, such as MacArthur, MacDavid, MacDonald, MacMillan, and very many more.
The P-Celtic Welsh map “son” has also been involved in surname-formation, although usually reduced to ap or more often just p or b. Uprichard and Pritchard were originally Ap Richard, and Parry was Ap Harry. Powell comes from Ap Hywel, and Penry from Ap Henry. Probart was Ap Robert, Price was Ap Rhys, and Pugh was Ap Hugh. Bevan indicates “son of Evan” and Bowen “son of Owen”.
From the Isle of Man, there are many Manx surnames which involve a similar truncation of the Q-Celtic mac, this time to k-. Kermode was originally Mac Dhiarmada, Clague was from Mac Liaigh, Corlett from Mac Thorliot, and Quayle from Mac Fhail “son of Paul”.
Epistle, a literary or these days humorous word for “letter”, comes from Latin epistola, which was originally taken from the Ancient Greek form epistolé “message”. This was composed of Ancient Greek epí “upon” plus the verb stéllo “send”. It was borrowed into Old English very early on, and then later borrowed again from French.