[Verginius Rufus]

I think there is something in Caesar's Gallic War about Gauls (in what is now France) using Greek letters to write their language.

Acts 13-14 has information on Paul's travels through Asia Minor that could be part of the Roman province of Galatia. (I haven't checked a map to see the Roman provincial boundaries.)

The letter to the Galatians is in Greek and obviously not directed to the Celtic Galatians but rather to the "foolish Galatians" (3.1) who must obviously have been non-Celtic.

[muawiyah]

There's a wealth of information about languages used in Gaul. There had been a Greek colony settled there near what is now Marseille. In fact the current town, founded about 600 BC, was known in antiquity as Massalia (from Greek: Μασσαλία).

You can't keep that sort of thing hidden, and since it was a commercial center there were always some well-educated slaves around who could be sold to Celtic seacaptains moving hides and other commodities from Northern Spain and the British isles to market elsewhere in the Mediterranean.

The Galacians in Northern Spain have their own records regarding the departure of Galacians, with their Basque slaves, to conquer Ireland (then called Scota).

The English always dismissed those records as fairy tales because, alas, they were written in Greek and, as everybody (English) knew, there were no Greeks in Ireland!

I don't think the Brits thought all that far ~ and with a serious Greek Colony in Southern France, it was a major loss to archaeology during all those years the Brits dominated the field.