In the calendar, why is one era named in English (B.C.) and one in Latin (A.D.)?
Because our current calendar system is based on the Gregorian calendar with an English influencehttps://en.wikipedia.org/wiki/Anno_Domini
English is Before Christ, (or less religious B.C.E.; Before Common Era). A.D. is Anno Domini, which means in AD.
BCE: Before the Common EraCE: Common Era
BC=before Christ AD=after death or after Jesus died