Technically there is no such language, since such a language would be a universal language - which needless to say does not exist. However, there are languages which are common enough that one can travel most (not all) places and find someone who understands them (so long as they stay in the cities, ayway).
At present that language is English and - to a somewhat lesser extent - Spanish and perhaps Arabic. Neither of the latter are popular in terms of primary speakers, but so many countries have one or the other as a first or second language that I feel they must be mentioned. Especially as English's call to fame is not its number of countries where it is the first language, but instead the number of countries where it is the second language - or is commonly taught due to that country's common interactions with other counties that use it (such as in China and Japan).
In terms of number of speakers, the 'common' language would be Chinese, but it is limited to one country and few others use it as a second language or even teach it commonly unless they are near to china (such as Korea, Japan, and some countries in the Indochina region).
It used to be French (during the late 1700s - mid / late 1800's, I think). During much of the 1800s, if I recall correctly, French was known as the Language of Diplomacy because of its commonality. It was followed by English in part because of the British Empire and in part because after the Empire's dissolution America (another English speaker) happened to be on the rise.
Before that it was either Latin, Arabic, or Chinese - dependent on whether you were in (most of) Europe, the Islamic World (from Morocco and southern Spain along North Africa, through the Middle East and Persia right up to parts of India), or the Far East (China, Korea, Japan, and some parts of Indochina and Tibet). India had two or three major languages at this time and several minor languages. Of course, it was also several dozen countries at this time - much like modern Germany and Italy used to be - but over more land and not sharing a common language.
Most other parts of the world (prior to the 1500s - 1700s) had an amazingly high language density (North and South America, Africa south of the Islamic World, most islands in the various oceans). I'm not too sure about how many were in Australia during the Middle Ages, but I presume it was as dense - or nearly so - as in the Americas.
There is also Eastern Europe to consider - at least during the mid to late middle ages. It was mostly under the influence of the Orthodox church, and I don't know if they had a proscribed language for their religious services or not. Perhaps southern europe - mostly under the influence of the Greek Orthodox church - required Greek, while northern europe - mostly under the Russian Orthodox church - required Russian. Either way, it would have been a bit of a sqeeze in the south, as the Islamic World, towards the end of the middle ages, managed to take a notable chunck of southeastern Europe for itself. So Greek may not have been common enough at that time to be considered a 'common' language in the sense it is often used in D&D.
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Having said all that, Chinese is important in the far east to this day but elsewhere in the world it will avail you not. Spanish is king in terms of countries where it is first or second language, but most of them are in Americas. In the rest of the world - outside of Spain and a few islands - it will avail you not. French is common in most islands in the Atlantic and Pacific oceans due to early explorations and its commonality (as the language of diplomacy) at the times of the explorations, but outside of France and those islands (and a few other places, like French Guina in S. America and Quebec in Canada) it will avail you not. English is the first language of America, Britain, Australia, and co-first language in Canada (with French). Its claim to fame and fortune is - no pun intended - as a trade language.
Much like 'common' in D&D games, English is commonly known in many lands not due to being the first or even a secondary language of the land, but instead due to being a common trade language that most involved in international business can be expected to know (or readily be able to find someone that knows it). Even those not involved in international business, but large enough to have a major national business likely have staff that speak English - if only for speaking with suppliers of parts from outside the country or should they consider expanding internationally later.
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Now, here's a new question for you:
How many actually treat languages as skills, requiring more than one rank to fully learn? I typically treat them as normal skills, but I grant a +8 bonus for homeland languages - treating racial languages as such if more than one language is known. Thus anyone from the homeland region - even with no ranks in languages - can speak it and understand it with ease (albiet with a strong accent if they have not put a rank or two into it). Being able to speak a language grants one a +4 bonus to attempts to read the language. Languages with common alphabets grant synergy bonuses to each other - at least in the case of reading or writing them.
To balance this, I grant a couple extra skill ranks per level that can only be placed in languages or knowledges (and everyone has Knowledge (local) as a class skill, albeit needing to place ranks for each region, of course, and Know (this region) will grant a synergy bonus to understanding the main language of said region).