How do you pronounce celery?
Keltic is new as of about 30 years ago, Seltic is the proper English pronunciation.
The OED gives both. Keltic is what every ethnographer I’ve ever heard says. I’m going with Keltic. You are welcome to be an outlier
I would assume that Seltic is correct in that the sports team beloved by every Irishman Glasgow Celtic is also pronounced Seltic, not Keltic.