(A good recent book on the linguistic history of the British Isles is Glanville Price, ed., Languages in Britain and Ireland, Blackwell, Oxford, 2000.) The Celtic languages, although ultimately they are distantly related to the Germanic languages and hence to modern English — Celtic and Germanic are two branches of the “Indo-European” language family — are very different indeed from English.They are at least as “alien” as Russian, or Greek, say.If we went back one thousand years and heard the language spoken in England then, we would not easily recognize it as connected to our own, modern, English.We could not understand it without a course of study, as one studies modern foreign languages.A little further back, West Germanic speech had split off from North Germanic, the ancestor of modern Danish, Swedish, Norwegian, and Icelandic, and from a now-extinct language called Gothic which formed a one-member East Germanic subfamily.Shortly before the time of Christ, the ancestor of all these languages was a single language that modern scholars call “Proto-Germanic”.If we go back to the beginning of the Christian Era (that is, the years around A. 1), shortly before the Romans conquered Britain, then the inhabitants of Great Britain spoke Brittonic or Brythonic, a Celtic language, ancestor of modern Welsh.(In northern Scotland a language with another name, “Pictish”, was spoken; little is known about Pictish — it went extinct long ago — but the best guess is that it was simply the northernmost dialect of Brittonic.) The inhabitants of the neighbouring island of Ireland spoke a related but rather different Celtic language, Goidelic or Gaelic.
A language can die, if it is used by fewer and fewer people until it has no speakers left; but languages are not born.But after Scotland was united with England in 1707, Scots faded away except as a local spoken dialect among many others in the English-speaking world.) However, if we were to go back another thousand years, from A. 1000 back to the time of Christ, we would find that Old English had emerged from splits which turned one language at the earlier time into a number of later languages requiring distinctive names.Old English shared a common ancestry with other languages that linguists describe collectively as “West Germanic”, including Dutch and German.In reality, though, by the time of the collapse of the Roman Empire, Latin was spoken across a wide area of Western Europe, and different changes to it accumulated in different territories.Present-day Italians, Frenchmen, Spaniards, Rumanians, and others all speak languages which have evolved gradually out of Latin, but they do not speak the same modern language; so they need distinctive names for their languages: Italian, French, and so forth.