In today’s English, it’s pronounced Kelt. It may once have been differently pronounced but most words were pronounced differently long ago. To quote Blake (from memory, so...)
“A horse misued upon the road
Calls to heaven for human blood”
That rhymed 200 years ago.
How do you pronounce celery?
Keltic is new as of about 30 years ago, Seltic is the proper English pronunciation.