For the oldest written record of English Common Law, that would probably be Italian. The Romans made several observations about the nature and functioning of the curious legal system used by the barbarians in the isles before the birth of Christ, as did many other ancient scholars. Compendiums of the case law were not created until much later but were based on ancient precedent, written and unwritten. For the clueless, most of the rest of Europe (and Scotland) uses the historical Christian legal system.
It is kind of obscene that Christians "claim" English Common Law because of its arguable superiority despite unambiguous pagan origins, and conveniently ignore the major legal system of Europe which is essentially Christian in origin.
I think you might mean "latin." And the "pagans" in the mythical Dark Ages you suggest (I guess pre-400 AD?) weren't sufficiently organized to have much of any law beyond the tribal and primitive. Literacy would drive the organization of law.
I don't really understand your gripe--and I don't hear much from Christians about English Common Law one way or the other as any kind of "argument to authority".
The English got their legal traditions much the same way as the rest of Europe--from the influence of the Roman Catholic Church, it's power and clergy. But the English colonized the US, so our law system would be informed by the English style.
"LOL" indeed!