Ray, Stephen & all.
Yes, St. Jerome does mention that in his time the common people of
Galatia still spoke a language nearly identical with what he heard as a
youth about Trier. There are also apparently some (Greek or Latin)
inscriptions of the third century, whose subjects are said to be
descended from the Galatian Tetrarchs.
In a couple of web sites I've found it said that a Celtic language
survived in Galatia until the Eighth Century. No references were given,
& I'll withold judgement until I see something authoritative.
Another interesting question is the degree of communication between the
Galatian tribes and their kin elsewhere. The Tectosages and the
Tolistobogii both had "branches" in southern Gaul. Were the Trocmi
(Trocmes) also represented outside of Galatia? In 196 BCE when the
Galatians attacked Lampsacus, the government of that city, through the
good graces of Massilia, got a letter from the Tolistobogii of Gaul
asking their Galatian brethren to make peace. That was 80 years after
the Galatians entered Asia Minor.