“Habemus Novam Dominam Sithi” – Is this the correct translation to Latin for “We have a new Sith Lord?” I know this seems like an odd question. It was motivated by the fact that the new ‘pope’ looks kinda like the Emperor from Star Wars (with the bags under his eyes &c.).
It should be “habemus novum dominum” - otherwise you’re talking about a female (domina) 
You may have been misled by the word “papa”, meaning pope. It is, despite its -a-ending, masculine. I don’t think it’s classical Latin: I know the word occurs in Erasmus.
By the way, a pope is also called “summus pontifex”: a pontifex is, as the word suggests, someone who builds bridges (let’s hope Benedictus can do that!!!)
About the Sith part I must admit I don’t know 
But if the name is “Sith” I reckon a Latin version could be “Sithus” and the genitive as you use it could be “Sithi” 
I think the gen. pl. is better, unless I misunderstand the meaning of Sith: Sithorum.
It’s a very old word and can be found already in Tertullian. I assume in church texts it originally used to mean “bishop.”
Thanks Deses,
A bishop is, of course, also called an “episcopus” (Hi, Episcope, if you read this!) Or a chorepiscopus… And there are clerus and ecclesiasticus and perhaps other titles…
I think the Romans’ religion is also quite interesting. They had sacerdos, antistes, flamen, sacrificulus, fetialis… Does anyone know about the differences among these different religious professions?
Thanks! Yeah, I forgot about gender. I just copied “Habemus papam” and kept the a without thinking.
In Eccles. Latin pontifex means high priest as in Hebræos 3:1 “Unde, fratres sancti, vocationis cælestis participes, considerate Apostolum, et pontificem confessionis nostræ Jesum:” [Therefore, holy brethren, partakers of the heavenly vocation, consider the apostle and high priest of our confession, Jesus:] – That would make pontifex maximus and summus pontifex mean highest high priest. Certainly then there is an implication by the pope that he is higher than Jesus.
Papa means father. It was applied to bishops around 250. In fact the Roman clergy wrote several letters to Cyrpian bishop of Carthage where they call him papa. Later one of the popes banned the title being applied to any other bishop. Luther used it in his 95 theseis of course, # 86. Item. Cur Papa, cuius opes hodie sunt opulentissimis Crassis crassiores, non de suis pecuniis magis quam pauperumfidelium struit unam tantummodo Basilicam sancti Petri?