[frithguild]

The Picts also inhabited the Lowlands. They spoke Celtic but were likely of pre-Celtic origin, sometimes referred to as the "auld black breed."

If the Picts spoke Celtic, why did St. Columba need a translator?

[Wallace T.]

Just as you would likely need a translator if you were speaking to a Norwegian if you didn’t know Norwegian and he didn’t know English, even though both are Germanic languages. There are enough differences in members of language families that translation is required between their mutual speakers.