In the real world it's common for people in Slovenia to speak Slovene, but once you get across the border it's common for people to speak Italian. So you could theoretically have Language (Common: Slovene) and Language (Common: Italian) but then what's the point, right?
Nowadays I can go almost anywhere in Europe and people will speak English. It's the common language of international communication. Other big languages are Spanish, Chinese and so on, but English is pretty dominant, at least in the part of the world I live in.
Now, it never made sense to me why Common in D&D was basically considered the language of the commoners everywhere. It's not conceivable. If we take that the basic trait of the "Common Language" is that you'll be able to speak it "anywhere" then the best medieval example would be Latin. In basically ever village people would be speaking their own dialects and languages and nobody really understood each other beyond their immediate geography. But everywhere in Europe people spoke Latin. Except they weren't commoners but priests, scholars and so on.
So if you learn common, you will be able to communicate anywhere, but you'll have to find a specific person who knows it. You can learn specific languages, but their application is limited. If you're a knight from France, you'd be hard pressed to find someone who speaks French (or perhaps more accurately Occitan) in, say, Prague. But if you know Latin just find a priest and you're good.
That's the sort of common language that makes sense to me.