1. "Kel-tick" (with a hard "k" sound): This pronunciation is more common in American English and parts of Canada. It reflects the historical influence of Latin, where the "c" before "e" or "i" was pronounced as a "k" sound.
2. "Sel-tick" (with an "s" sound): This pronunciation is more common in British English, Australia, and New Zealand. It reflects the historical influence of Old Irish, where the "c" before "e" or "i" evolved to be pronounced as an "s" sound.
Essentially, the differences in pronunciation boil down to different historical sound changes that occurred in the evolution of these languages.
It's important to note that both pronunciations are considered correct, and you shouldn't be penalized for using one over the other. However, it's generally advisable to use the pronunciation that is common in your region or the context of your communication.