How exactly did Gilbert get onto a railway platform with a horse?
Perhaps it's just the scanlations that I've read, and perhaps it's correct in the Japanese versions, but it seems like almost none of the Latin makes any sense whatsoever. It certainly doesn't say what it claims to say. This wouldn't be so bad, except when Greek is used in masses, it's used correctly. Would it really have been so terribly difficult to use correct Latin, in the places where Latin appears?
The Latin is indeed the same nonsense text in the Japanese version. I couldn't tell you why, but it's pretty common for foreign languages in manga to appear as dummy text.
Just what is Jules's problem with Serge?
This is pure speculation and I can't read Japanese but here's my explanation: Jules cares about Gilbert a lot, to the point that he feels a sense of responsibility to "fix" him. Jules being Jules, his method of choice involves giving Gilbert a lot of space, carefully-timed assistance, and subtlety. Serge also wants to help Gilbert, but he does it in a much more direct and naive manner. Serge essentially ruins Jules' plans, and causes Gilbert to change more quickly but also takes more risks that could make things much worse. And thus, Jules sees him as a potential threat to Gilbert and the ways in which he is trying to take care of him.