'Common' language, sometimes under a different name, is a very persistent element of many imaginary worlds.
It has some ground in history, with various lingua franca languages, but in imaginary worlds Common usually is far more ubiquitous and widespread than any lingua francas would, up to being a default and only language of many different cultural groups, default and only language of many different political powers, default communicative language between distant and isolated locations, or used by monsters, even without physical capability to actually speak this language.
Sometimes the world justifies Common in-history, such as in Greyhawk and Golarion where Common is specifically the language of somewhat-recently-widespread empire(s), but most of the time the Common assumed to be a sort of potluck language that came into existence somehow on its own even in the grim points-of-light worlds that didn't have such widespread recent historical notions.
A language is also something that naturally and continuously changes over time and distance: French language spoken in Quebec isn't the same as French language spoken in France, and English, even so widespread and ubiquitous, has a hundreds small differences depending on the area it is spoken in. But there is never 'Old Common' language, neither there are any regional dialects, even in the case of far and isolated realms. It comes to somewhat absurd degree in settings such as Planescape, where beings from
different worlds and planes of existence speak the same Common language
just fine. It is Common everywhere, and the Common just is, as a cohesive and unchangeable whole, unnaturally resilient to being anything else but the Common.
English is my third language, with first two being close enough that they share a lot of structure and word-roots but different enough that each one has to be learnt separately. One thing that strikes me is how it is often impossible to translate both poetry and jokes from one language to another with 100% precision, without either substituting some close – but not exactly the same – words or idioms, or losing the wordplay, the rhyme, the meter or punchline. It is downright impossible sometimes to translate a song from other two languages into English without making a page of commentary on what this word salad supposed to generally mean because English has more rigid sentence structure where a sequence of words is treated differently then a sequence of words in languages of more lenient structure. But yet, in-games with Common, even if hired translator is used in those rare cases when Common isn't a local language, their translations are supposed to be 100% precise and trustworthy, and without losing the meaning or the punchline.
Common is in this way because it is gameplay, not in-world, necessity: if my experience is of any measure, it is extremely rare that people come to play tabletop RPGs with intention of dealing with linguistic intricacies. Players want to participate in whatever action is going on, and in the real world the language barrier is one of the most powerful things preventing this(*). Thus for the sake of fun and convenience such intricacies are omitted, and player characters are given 'Common' as a mirror of whatever language is used at
the gaming table. To further make the language a non-problem, 'Comprehend Languages' is one of the most basic spells to fill the gaps in understanding where Common isn't enough, and, as another nod to the gameplay, the full language mastery through 'Tongues' is just low-to-mid-level-spell, to eliminate a language barrier entirely as long as a mage is around.
(*the language barrier can be a good tool to create a sense of alienation, social helplessness, and maybe even horror in some settings or games. There is only so much one can perceive from a tone or a body language if they don't understand what is going around them. Trustworthiness and precision of translators can also play a role in social intrigue type of games.)
But thanks to this gameplay convenience, when taken in-universe Common as a language has a lot of unnatural features, and thus can be seen as self-propagated, self-sustained, monolithic mnemonic entity, a memo-virus similar to the Idea of Thorns from Gardens of Ynn, only much more subtle. Such memo-virus would be quickly replacing in-world languages as soon as it is introduced to/infects the local populace: it is just too darn inconvenient not to use Common, and thus local languages and dialects die within generation or two, replaced by it, with intricacies of previous languages lost without native speakers. Being a subtle memo-virus without counters, Common can maintain its structure unchanged, without necessity to adapt; maybe it is even lazy enough to share the space with other languages as long as it is in the mind of the speaker and the most dominant one used.
Regardless if Sapir-Whorf hypothesis true or not, poetry in particular is vulnerable to translation. As Common becomes more and more prevalent in each society, the poetry becomes written mostly in it and eventually more and more homogenized comparatively to what it could have been. Taking this idea even further, in a world where the magic source is the chaos of imagination, lured in and barely contained in the frames of meter and rhymes in the minds of the poets foremost (and only then, through effort, translated, distilled and brought down to useful, functional mundanity of spells by science-wizards and their dry practical memorizational minds), Common as a dominant memo-virus can also explain the uniformity of spells; without other languages to give poetry some different frames and angles, in the world of utter Common dominance the magic eventually will reach the limit of what can be expresses in it.
And at the last, one can treat Common not as simple mindless memo-virus, but as an Eldritch God, a formless being propagating through the minds of mortals and immortals alike, influencing their ways to express themselves and thus affect the world to its liking.
P.S. I am also finding its name, "Common" to have a lot of rather sinister depth if one would like to play up the themes of homogenization, what is perceived as normalcy, lowest common linguistic denominator while taking Sapir-Whorf hypothesis as a truth for some given world.