Inspired by this question. I was thinking about the constructed language I'm building and a group of countries in the history of my world, and ended up wanting to ask this question. Please do take the time to read all of the information under the main headers of "Info" and "A Little History Lesson" and think for a minute on the information contained in the subheaders. It will be important for answering the question and understanding the setting.
I have a small little country, about the size of modern day Portugual, called Teyarzhaodaqua (From Teya-Řãonance-de-Aqua, lit, "The sun's domain of water") bordering (the longest border exactly parallel to) a country the size of modern-day China, called Aeðsta. Teyarzhaodaqua is not technologically or magically advanced, has very little education or status, and has few resources and weapons. Aeðsta is highly technologically advanced and has plentiful weapons and resources, lots of education, fine arts, sciences, and many people there are wealthy, as well as powerful mages.
Now, I have a scenario in which the language of Teyarzhaodaqua (From now on referred to as LC) becomes the official language (the only language spoken, surpassing the original language) of Aeðsta (from now on referred to as BC). That is to say, the language of LC completely replaces the native (and sole) language of BC (English).
Background on languages
The language of LC is hard to learn (at least for speakers of English, like BC), with an alphabet containing more than 75 characters (26 base letters plus 50 diacritics), difficult pronunciation, and confusing temporal reference system due to verb conjugations among ten sets of pronouns, and occasionally additional meaning is added to the written form by switching the color of the ink, which makes it a pain to write as well. It's not magical and doesn't have any kind of power/religious significance (as a matter of fact, it is occasionally viewed with scorn, since it has no religious significance), takes much more time and effort to learn, is a pain to work with, and isn't really connected to, say, important documents, scholarly findings, ancient works of literature, valuable information, etc. It's not economically important (not learned for trade) and not important for peace talks between the nations (LC'ers learned BC's language quickly, enabling communication.) So it's not important for diplomacy/money. It is a beautiful, melodic language at times, but the pronunciation of some words is alien, and occasionally harsh.
Important people, and all that stuff
The government of BC is a representative republic with three different law systems, each with an elected council: Law makers, law passers, and enforcers. There was no language barrier between the upper and lower classes of BC. LC is an oligarchy ruled by a group of spiritual leaders who make pass all laws within a small council.
A little history lesson
The countries of LC and BC are nearly completely isolated from other civilizations due to being a landmass out in the middle of an ocean. LC was the original country that did not cover the entire landmass, but when BC came it conquered LC and all unused/unexplored land. BC was essentially one tribe that came over with one language. What makes this scenario interesting is that LC is literally cut off from time. All of the important technological developments of the rest of the world--from the invention of the wheel to the invention of gunpowder--took much longer to develop. LC's tech level is similar to that of Medieval Europe. BC, on the other hand, has 19th/20th century tech.
Barrier of Nature
Geographically, the little continent where BC and LC are is isolated by a formidable wall of defenses, namely a ring of jagged rocks made of graphite that has been compressed under heat, pressure, and time to create diamond "teeth" that form a vast barrier around the continent. Strong currents mean ships are pulled against the rocks with enough force to smash through wood. Past the diamonds, on seemingly dry land, there's a high wall of rock followed by a sudden depression. When tides are at their highest, the water on the other side of this wall surge over into the depressed ring--a death trap for survivors who paused to catch a breath. And if you make it past that? Say hello to poison elephants, trees that stab you, and carnivorous silkworms, as well as snakes whose venom turns flesh to stone and vampiric flowers.
Last bit of background
The local inhabitants, LC, having migrated from another plane of existence (the Light Plane, specifically a planet with 27 suns) have been eking out a living. And now, a couple thousand miles away, a fleet from BC approaches the continent . . . and survives.
The two communities are separated by a large mountain range that severs the land into a large chunk and a tiny chunk (that will later mark the boundaries between the countries.) BC is already at the medieval level, but LC is primitive.
The two countries develop, isolated from each other and the rest of the world, until they reach the level discussed earlier and BC looks up from all the stuff it's doing and wonders, hey, what is on the other side of those giant, treacherous mountains? And so we come to our scenario, where peace has been (grudgingly) made between the countries, (though conquest may still come) and they are exposed to each other's culture and language for the first time. The paths between the mountains are opened and . . . suddenly they are minutes away due to BC's communication technology.
Relationship Between BC and LC
BC's opinions of LC: "LC is inferior. It has less advanced technology, a weak government, a blasphemous sun-worshiping religion (can you believe they don't worship stone!?), a difficult language, strange customs, and to top it all off they're using up valuable resources and living on our land. But on the other hand, their language is connected to the language of the Light Plane, which is a potentially valuable resource, considering it might be useful for reading the magical tomes in our possession. Perhaps a temporary alliance would be useful. Our religion demands tolerance of other's beliefs, even if they are *sniff* blasphemous."
LC's views on BC: "BC is obviously inferior. They worship stone instead of the sun — how can that be forgiven? They speak a harsh, hard, primitive language while trying to make up for it with their "technology." Well, we'll show them how quickly we can adapt! And they can try all they want to take our land from us and change our religion — they'll regret it. But we need to try to be peaceful for now, since their technology and sciences might be useful to us."
Question (Hey, congrats on making it this far!)
What sociological event or trend (that have happened in our (real world's) history in similar situations) could have made BC make LC's language the lingua franca? It must adopt both the complex written form and the spoken form. It must become both the official language (what's taught) and the only spoken language (what's actually spoken in the street.) A number of "uneducated souls", like street urchins/criminals/poor people, won't speak much of it, but they have to know enough to get by. This doesn't have to happen immediately, but it has to happen over a period of about 50 years without any major upheavals in religion, government, etc.