My derailer (or derailleur, if you like): my Brit BIL taught me to say “Tems”. When I moved to Baltimore I moved right onto Thames Street. Took awhile to get the hang of alternate pronunciations.
Dan
And, to pick another nit, “Celtic” isn’t a proper noun. It’s an adjective - the adjectival form of the noun “Celt”.
In the Gaelic languages (Irish and Scots dialects), the letter “C” only makes one sound when followed by a vowel, which is the K sound. (When followed by an “h”, obviously, it changes into the clearing-of-the-throat sound).