This probably only part of the answer, but classical literature (including theological writing in the case of Christian Europe) played a large role in creating this situation, and cultural exchange between people with different native languages helped maintain it. Something similar happened as well with Sanskrit and Classical Chinese.

Here's the outline of how this process goes, although obviously details vary. I've provided some examples for Greek, Latin, and Chinese; I'm not familiar enough with Sanskrit to do the same for that language, but I believe the general pattern still holds.

• A certain people's economic and military prowess (e.g. the Macedonian empire) causes its language and literature to be seen as high culture by those within its sphere of influence. Because culture spreads more easily than empire, this cultural sphere of influence ends up being a bit larger than the economic or military spheres that caused it, including places (like Japan or Scotland) that were never under direct control.

• As a result of this perception of high culture, educated people throughout these spheres learn the language so that they can read its literature.

• Since all educated people, regardless of native language, understand this dominant language, it's also a convenient way to communicate new ideas to a wide audience. So if you want to get your idea out--whether that idea is Newtonian physics, the story of Jesus, or the basis of number theory--you write it in this dominant language.

• All the works listed above were written by non-native speakers in a dialect whose primary purpose was communication between peoples, rather than day-to-day vernacular use. The dominant language has now become a classical language, one we learn because famous things are written in it, and write in because everyone else has learned it.

• Since language-learning takes time and effort, and learning a classical language is only really useful to people who have time to read and write, it tends to be restricted to the upper classes and thus eventually becomes a mark of those classes.