It’s a bit complicated. So the word CAESAR in English shows some anachronisms, it was borrowed from Latin but it’s pronunciation reflects the pronunciation of medieval-era French loanwords and in French as well, it shows some oddities, specifically some attempts to pronounce it in a more archaic way. I’ll try to give the broad details without getting too lost in the weeds.
Just to clarify, I’ll use IPA, the phonetic alphabet, the consonants mostly align with English, but English vowels are way off. The IPA vowels I’ll be using reflect more how the vowels are pronounced in Spanish or Italian. So Caesar in English is /sizər/ (or /si:zar/, the : indicates a long vowel and that varies in English).
In Classical Latin, CAESAR would’ve been pronounced /kaesar/, pretty much exactly how it reads, but as time went on, Latin, like all languages changed and eventually evolved into the Romance languages.
In the Late Latin period, three big changes happened 1. /ae/ disappeared and became the sound /ɛ/, much like the sound of English bed.
- /k/ before the sounds of /e/,/ ɛ/, /and /i/ became /ts/ in what is now France, Spain, and Portugal. This is what caused the split between hard and soft C.
- The /s/ sound in the middle of the word became /z/ in between vowels. So by the fall of the western Roman Empire, CAESAR was written the same but it would’ve been pronounced /tsɛzar/.
The /ts/ eventually became just an /s/ in French. Though in Spanish from Castille it ended up as /θ/, like the in English thin.
Anyway, this influenced English as well. English didn’t exactly borrow it from French, but the pronunciation patterns that give us the pronunciation /sizər/ did come from French. In Middle English, it would’ve been pronounced something like /se:zar/ (/e:/ is a long version of /e/, like the Spanish ).
However, /e:/ in Modern English became /i:/, you can compare the pronunciation of the Old French loanword feast /fi:st/ to the more modern Latin loanword festival, which more or less reflects a more traditional sound as it was borrowed after the shift.
The final vowel is pronounced /ə/, this is just the sound of unstressed vowels in English, you can compare it to the in charity. And obviously the uses the English /ɹ/ or one of the other “r” sounds or rhotics in English. I’ve just used /r/ in IPA because that’s a whole nother conversation.
Now, there are some oddities. Mostly the sound of the AE, which by all means, should be /je/ in Modern French, like in the world ciel from Latin CAELUM or sky, but it’s /e/, the word is César, not * Ciesar.
The other Romance languages show similar oddities, Spanish César, Italian Cesare. What most likely happened is the name was borrowed directly from Latin later on and missed some changes or was only partially adapted to the language. This is not uncommon.
Additionally, he original Old English direct from Latin was cāsere, pronounced /ka:zere/, but it also competed with Norse derived versions and now, it’s more similar to an Old French loanword which has gone through the vowel changes that characterize Modern English.