There is no apostrophe in Kaldorei.
Writing it as “Kal’dorei” is what got you docked points by your Darnassian language professors and why they generally looking down upon you.
So then why are there apostrophes in Quel’dorei and Sin’dorei? One might think these were completely arbitrary and nonsensical uses of apostrophes in made up languages without any actually thought out grammatical structures.
But the answer might be found in English itself. Apostrophes are used for contraction. That is to say, an apostrophe can be used to contract the word for High - Quel - and Kaldorei to make Quel’dorei. Which is what the Highborne quite literally were, High Night Elves.
One might protest that there’s no way that Sin’dorei means Blood Night Elves, if one were to consider Sin’dorei a contraction of the word for Blood - Sin - and Kaldorei, and likewise that there is no way that Ren’dorei means Void Night Elves. But luckily Sin’dorei and Ren’dorei are not actually Darnassian words. They are Thalassian words. And, well, the Highborne were known for their arcane studies, not their linguistic ones.
We can see this was the case for the Nightborne as well, as they also put an apostrophe in Shal’dorei, but had failed their Darnassian vocabulary quizzes and mistook the word for Shadow - Shal - to be the word for Night, and were too embarrassed to admit this, so they said they were actually talking in an entirely different language, Shalassian, instead rather than admit their mistakes.
TL;DR:
Stop putting an apostrophe in Kaldorei.