I already knew there was no "w" in French to speak of and after looking at Welsh and Gaelic place names there is no "w" there either. That leaves Brythonic, Scandinavian, English, German, Flemish to name a few. Not being a language expert I find it hard to postulate here but it seems to me that Caldwell to have retained its spelling from such an early date must have come up through the Scots Lallans form of English. That dates it back to no earlier than the Anglian occupation of Strathclyde - gives us a bit of leeway still as that was pre 1000 AD

Tom