My question on rpg.stackexchange.com seems to have reached a point where a "history-person" would be quite suitable to answer it. So let me rephrase it, so as to be at least marginally suitable for this site.
Which were the continent-wide common languages during human history (I can think of English, Latin, Greek in reverse time order)? What percent of the populace spoke those languages? What percent of the literate populace spoke those languages?
Well, these days I'd say Spanish certainly counts. It is spoken as a first language in just about every country in the Americas south of the Rio Grande (Brazil being the most prominent exception). North of there, English has roughly the same status.
Historically, the best analog I know of is Mongol, which at one point was spoken across Asia from Russia to Manchuria (China too, but only by the rulers). I don't have numbers on 13th century Asian literacy, sad to say. I'd guess that few Mongols were literate. Their alphabet was brand new at the time of their empire, and being pastoralists by culture most of them would have had little use for it. Then again, your typical Chinese or European peasant didn't have much use for literacy in the 13th century either. (I should note here that these days Mongolia's literacy rate is a respectable 97.5%, which is quite a bit better than neighboring China, and puts them slightly more literate than Greece)
I'm going to introduce my definition of "Continental" size as an entity with at least 1 million square miles, and 100 million people in its modern population.
English is one such language, spoken in "North America," specifically Canada and the United States. Not to mention a number of countries that make up the former "India" and the current Indian subcontinent.
Spanish is spoken in the most of the South American Continent (except for Brazil). Portuguese is spoken in Brazil, which meets my definition of "Continental size.
Greek was spoken not only in Greece, but in the "subcontinent" of Asia minor, basically the empire of Alexander the Great. Ditto for Persian, in Asia Minor, when they ruled before Alexander.
Under the Roman empire, Latin was spoken in southern western Europe, enough of Europe to meet my definition of "continent."
Chinese is spoken in China, a "confederation" of land and people of Continental size. Russia, where Russian is spoken, is larger than most continents.
From the question:
You and I speak "Common" – it's called English. But this is the result of the recent globalization made possible with the advent of the Internet.
This is incorrect. It is the result of Imperial Conquest, and that, I think, is the real heart of the matter.
If you look at Common as an imperial language - an official language of government - then yes, human kingdoms at war and "non-humans" (and let's be frank, that notion is grounded firmly in Tolkien's quaint Victorian notions of race) would both speak it and their own language, even if the empire is a fading memory.
Take, for instance, not a continent, but two subcontinents: India and Europe.
In medieval Europe, if you knew Latin, you could generally find someone in town who knew it as well - clergyman or clerk - and you could fake your way through a conversation in a place that spoke a romance language, if you hit your language rolls. So, Latin as "Common" would still require a player to sink some stats in languages if they want to talk to random villagers.
In modern India, you have the language of Empire, English - if you do business or deal with the law or government, you speak it. You also have the language of faith - Hindi - which even non-Hindus learn in order to communicate with others in the community. Then you have twenty one "mother tongues" - languages learned from your mother, this is the official language of the place where you live. Of course, there are even more unofficial "mother tongues", the language of your ethnicity, of your social caste, of your particular village that's different than the province's.
So, you would have an imperial language or two, "Common1, Common2", and some NPCs might know one better than other NPCs, but most everyone would know a smattering of either. Mother Tongues can be then broken down by race (ethnic-centric language) and alignment (caste-centric language).
So the way to run the campaign is to make the players roll language skill to speak common, to see if they can actually communicate. Knowing another "Mother Tongue" fantasy language, elvish or evil, improves the ability to talk to those NPC's that might also know them, even in passing.
Jacques Cousteau has a story of his wife, French, trying to hail a Greek captain on a nearby yacht, and both parties are attempting to say hello in every language they know - and though France and Greece are only a few hundred miles apart, they wind up speaking in Japanese! In this way, learning languages should improve a character's ability to speak with others generally.
See also, Lingua Franca a "third language" that people who don't know each other's languages communicate in, and poorly.
Everyone knows Common. No-one knows common very well.
Well firstly, what do you mean by 'continent'? Is Europe a continent? Is India?
Remember that now lots of people in the same country (upper & lower class) speak roughly the same language. However that wasn't always the case. You can see this in some places where minorities who have very little power would not speak the language of government. e.g. serfs in the field speaking Old English and the Norman lords speaking Norman French.
Also, people in different classes/professions would know different languages. e.g. catholic priests and other educated people would know Latin, Orthodox preists might know Greek in the mediæval period. In later centuries, educated people might know French. But that doesn't mean the common man on the ground might know Latin or French.
Here is a list of SOME "continent wide languages in human history":
a. The Greek language was communicated throughout much of the Mediterranean and Black Sea regions beginning in the 700's BC/BCE. Greek was widely communicated throughout the Southern and Southeast European regions during ancient times.
b. With the rise of Alexander The Great, the Greek language truly became internationalized by reaching into Egypt & the Middle East regions, thus expanding Greek beyond Southern Europe & Anatolia-(present-day Turkey).
Latin: With the defeat of the Carthaginian and various Greek imperial dynasties, the Roman Empire also spread the Latin language to greater distances within much of continental Europe-(when compared with the Ancient Greeks). The Latin linguistic legacy lived beyond the fall of Rome and into the Medieval period.
Arabic: The Arabic language, has been since Medieval times, the national language of the entirety of North Africa, as well as a sizable portion of West Asia/The Middle East.
Spanish: Every country in South America-(with the exception of Brazil), has been a Spanish speaking state since the mid 1500's. Every state in Central America has also been Spanish speaking for nearly 500 years. Even a sizable portion of the United States, during our early history, was primarily Spanish speaking.
English: The English language, is probably, the most widely spoken language in the world during late Modern times and into the contemporary age. English is the central language of the United States, as well as the majority of Canada. English, is a second language in many parts of Europe and is the central language of Australia. The continental legacy of the English language was due to the massive presence of British colonialism during Modern times, followed by the worldwide presence and influence of the United States since 1945.
Russia: The Russian Empire, followed by the Soviet "Union"/(or Empire), had Russian as either a central or secondary language across much of continental Asia. Countries, such as Kazakhstan, Uzbekistan, Tajikistan, the Baltic countries and Eastern Europe-(during the Cold War), communicated in Russian either as a primary or secondary language.
Just to provide this widely interpretable question an answer that is specific to China, today there is a common language called putonghua that is based on the Beijing dialect of Mandarin. While millions of Chinese people speak the Beijing dialect as their first dialect, in much of China (see map) local people speak a dialect of Mandarin that is somewhat different from putonghua, and is at times not mutually intelligible. It might be like someone from Scotland learning to speak American English, and an American learning to understand people in Scotland. (I found it hard) In my experience, everyone in these areas under the age to 50 is also conversant in putonghua.
Furthermore, there are a bunch of people in China (~ 300 million?) who speak speak putonghua as a second language, with their first language being completely different from mandarin, i.e. Wu, Min, Gan, Xiang, Hui, Yue (Cantonese), Ping, etc.. Interestingly, some of these languages also have dialects that are not mutually intelligible. I recently met a person from Guangdong who spoke 3 Yue dialects, and two mandarin dialects, and English. Would this be 6 languages, or 3? It can be argued either way (he is a computer scientist).
Putonghua, along with simplified characters and Hanyu Pinyin, were standardized by the CCP regime in the 1950s with a lot of assistance from Soviet advisors who had a ton of experience with language policy and literacy. Some of you may be interested to know that the predecessor to hanyu pinyin, named Latinxua Sin Wenz, was developed in the Soviet Union to encourage the literacy of Chinese migrant workers resident in the Sibera from Shandong province. The motives of the CCP regime in enforcing the Beijing dialect nationwide has an interesting story, but it is too long, and to tangential, to be included as an answer to this question.
Prior to the 1950s, China had many older standardizations. I don't know too much about it, but this stack exchange question provides a bunch of details about standardized Chinese covers back to about 500 BCE.
