Either the books were written in ancient letter that were long lost, or it was written in our world's language. Either Chinese kanji system or the Japanese kanji system maybe?
to be fair, they are essentially in the Victorian era which goes from horse drawn carriages to jets, tanks, poison gas, and nuclear weapons in 110 years with every event rapidly swinging the world
Probably, but Kai better get some hardness outside of high-pressure situations or else--by the looks of this chapter, and even if those were girls and not a man--Shakuntiira is going to break his if/when that happens.