Sometimes i think about the idea of Common as a language in fantasy settings.
On the one hand, it’s a nice convenient narrative device that doesn’t necessarily need to be explored, but if you do take a moment to think about where it came from or what it might look like, you find that there’s really only 2 possible origins.
In settings where humans speak common and only Common, while every other race has its own language and also speaks Common, the implication is rather clear: at some point in the setting’s history, humans did the imperialism thing, and while their empire has crumbled, the only reason everyone speaks Human is that way back when, they had to, and since everyone speaks it, the humans rebranded their language as Common and painted themselves as the default race in a not-so-subtle parallel of real-world whiteness.
In settings where Human and Common are separate languages, though (and I haven’t seen nearly as many of these as I’d like), Common would have developed communally between at least three or four races who needed to communicate all together. With only two races trying to communicate, no one would need to learn more than one new language, but if, say, a marketplace became a trading hub for humans, dwarves, orcs, and elves, then either any given trader would need to learn three new languages to be sure that they could talk to every potential customer, OR a pidgin could spring up around that marketplace that eventually spreads as the traders travel the world.
Drop your concept of Common meaning “english, but in middle earth” for a moment and imagine a language where everyone uses human words for produce, farming, and carpentry; dwarven words for gemstones, masonry, and construction; elven words for textiles, magic, and music; and orcish words for smithing weaponry/armor, and livestock. Imagine that it’s all tied together with a mishmash of grammatical structures where some words conjugate and others don’t, some adjectives go before the noun and some go after, and plurals and tenses vary wildly based on what you’re talking about.
Now try to tell me that’s not infinitely more interesting.
I like the idea of Common being a pidgin language as described above – human cultures have a wide array of language Irl, so I assume they have them in D&D and other fantasy settings as well.
Which leaves Common to develop along trade routes and in temples, picking and choosing the words that work best collaboratively. Traders are going to speak a number of languages anyway, so it starts there and spreads as cities grow.
Undercommon, therefore, is also a pidgin language, but between the sentient races of the Underdark. Perhaps it sprang up on it’s own and eventually met Common and they two pidgins fused! Language in D&D is fascinating and not explored enough imho.