In my own world, a single empire united much of the known world at one time. This empire enforced a common language upon the people and was, over the centuries of its existence, fairly successful in actually making the majority of the population use it.
The empire fell, as all empires do, but an odd confederation of kingdoms rose up after it. These kingdoms were banded together under seven co-rulers and tried to keep their own identities but also had a need for a common language. Their languages were similar, anyway, since all had been part of the older empire, but had drifted in the time since it fell. Once again, a common tongue was instigated, though it was not a drastic change and most of the population of the nations could already understand one another to some extent, anyway, whether it was a greater or lesser extent.
Well, THAT confederation fell and humankind entered into the Long Night (corresponds to the Dark Ages of our own world). It fell due to a divinely-inspired catastrophy and the inhabitants of the confederation, those who lived, were scattered.
There was drift in the language up to the current time, of course, but most people in the main part of the campaign world can understand one another with effort. The languages aren't too disimilar to rule that they are totally different around the Inner Sea area that is the main part of my campaign world, but they might require some exposure to if someone wants to understand them without difficulty. It's more like some extreme dialects of English.
A real-world example is a Kuwaiti guy who lived on my dorm floor in college my earlier college years. He had little money beyond what was required to live in the US and go to college here and didn't have a phone in his room. My roommate and I allowed him to use our phone to call home. He ended up making the calls very early in the morning due to the corresponding time in Kuwait. Backing up a little, I must specify that he lived in Kuwait but was Indian by nationality. My roommate and I found a tape somewhere which was nothing but Hindu music and singing. We asked the Kuwaiti/Indian if he could tell us what they were saying. He replied that he didn't know any languages but English. We were shocked, of course. I replied that he spoke something other than English when he talked to his family. He told me that he spoke to them in English, just that it was differently accented and in a different cadence than I was used to. I listened to his next phone conversation and, wouldn't you know it, he was telling the truth. I had not originally had any idea what he was saying but, given the key that it WAS English, I understood what he was saying without much of a problem. He was speaking English with different accents on words, making it more of a tonal language than what it is, and was speaking it VERY fast and with odd inflections. Nevertheless, it WAS English and I went from 0% understanding to about 90% understanding just by realizing what he was doing.
. Although... elven generations are a LOT slower than human//halfling/dwarven generations. It took a few hundred years (and tens of generations) for Latin to become the Romance tongues... and tens of elven generations is thousands of years.
