To take the real world as an example, humans from France would get French, humans from Germany would get German, humans from Italy would get Italian... in other words, the automatic language for Humans should be 'Local Dialect,' not 'Common.'
'Common' as a universal language is a convenience that by its very nature makes the inclusion of other 'living languages' in the game virtually pointless. If everybody can speak Common and communicate any concept 100% effectively, then the only practical purpose of other modern languages is to conduct clandestine conversations--and the more languages any given individual knows, the less useful it becomes even for this. Not only does this defeat the purpose of modeling languages in general, it's also hopelessly unrealistic in a world where the average person doesn't travel more than a few miles from their birthplace.
To put it in perspective, the closest thing to 'Common' in the real Medieval/Renaissance world was Latin - a language imposed on a large area by conquest, which over a relatively short period of time evolved into a series of unique languages, until it came to the point that nobody--not even its originators--actually spoke it as a primary means of communication anymore. It continued to be used for a time as a trade language, but it was something that people learned later in life because it was a necessary convenience, not because they grew up hearing it around them on a daily basis. 'Common' should be much the same - something that merchants and adventurers pick up because it enables them to conduct their business. Something like that works in a setting where there was once an imperial power that imposed its speech on its principalities, but which has subsequently declined.
However, as it stands, 'Common' is more like modern English - something that a large proportion of the world is being taught to be bilingual in from an early age. This is not a reasonable assumption for a medieval milieu because the only reason English is so widespread today is due to its preeminent position in the media. No global media infrastructure, no worldwide pressure to adopt the language it operates in, no Common.
I'd prefer if the language system modeled something like you see in The Thirteenth Warrior - at first they communicate in Latin (Common), but over time the outsider learns his companions' native language by listening to it. In game terms, parties make sure that they have at least one other language in common besides Common, and pick up others as necessary. That way the choice actually becomes pertinent, instead of an infrequently-used curiosity.