Lots of times, I see people using "Jedis" and "Siths" (I also use that), but it doesn't feel right. Or, maybe, it's correct.
Is there official word on this? What's the correct plural of Jedi and Sith?
Mace Windu: ...you must realize there aren't enough Jedi to protect the Republic. We are keepers of the peace, not soldiers. -- Episode II
It's difficult to find conclusive proof that the plural of "Sith" is "Sith" because (a) there are so few Sith and (b) the plural is usually given as "Sith Lords". These are the best I can find in the scripts:
Yoda: Destroy the Sith, we must. -- Episode III
Anakin: The Sith rely on their passion for their strength. They think inward, only about themselves. -- Episode III
Yoda and Anakin may be referring to the Sith Order rather than multiple Sith (Sidious and Vader in Yoda's case). In Anakin's case in particular, though, he uses the plural "their", "they", and "themselves" in the two sentences, so it's quite possible he is referring to individual members of the Sith Order. It would also make little sense to say that the Sith Order has "passion for their strength", so it's more likely he's referring to individual Sith members.
Also, this archived page from starwars.com describes the Sith and says:
Power-hungry Sith practitioners fought amongst themselves and dwindled their numbers. Weakened by infighting, the Sith were easily wiped out by the Jedi.
Again, this may refer to the Sith Order rather than multiple Sith, but the previous sentence refers to Sith practitioners rather than the order.
As @Richard noted in a comment, there is no use of "Jedis" or "Siths" in any of the scripts (even in the non-dialogue scene descriptions).
The correct answer is “Jedi” and “Sith”.
Obi-Wan: But he still has much to learn, Master. His abilities have made him... well arrogant.
Yoda: Yes. Yes. A flaw more and more common among Jedi. Too sure of themselves they are. Even the older, more experienced ones.
Obi-Wan: You were the chosen one! It was said that you would destroy the Sith, not join them. You were to bring balance to the force, not leave it in darkness.
Anakin Skywalker: [shouts] I hate you.
Pre-Visla: You're not Jedi, so what are you?
Darth Maul: We are Sith.
Pre-Visla: Do you serve Count Dooku?
Darth Maul: I serve no one.
Pre-Visla: I thought there can only be two Sith: a master and an apprentice?
"The Jedi" refers collectively to the group. You can probably go either way with the plural: "There are 3 Jedi(s) in the next room" / "There are 3 Jedi masters in the next room".
"Siths" sounds like you have a speech impediment.
And don't quote Yoda here. Bad example his grammar is, mmmmmyesssssssss.