Buying bread from a man in Brussels
He was six-foot-four and full of muscles
I said, “Do you speak-a my language?”
He just smiled and gave me a vegemite sandwich
—Men At Work, “Down Under”
The “Common Tongue” is one of many old-school D&D conceits: a single widely-known language that all humans speak. Depending on what historical era you’re using as a template, this isn’t necessarily far off the mark; Latin was certainly a world language in Roman times, the term “lingua franca” was coined for a reason in France’s heyday, and English is itself an effective common tongue. On a more regional scale, there have been dozens of languages that served as common tongues in some smaller segment of the world.
But what about non-humans? Do orcs know Common? What about gnolls, ogres and lizard men? Or the game’s eponymous dragons?
By the book, dragons capable of speech always know the Common tongue, while 20% of all other talking monsters also know Common. As such, any sizable group of intelligent monsters is almost certain to have at least one member that can communicate with the party. This makes knowledge of specific monster languages less essential to the party for purposes of communication (though still useful for listening in on their private conversations). It also makes negotiation a lot more feasible, providing a reliable alternative to combat for parties thus inclined.
(Conversational monsters also automatically know their alignment tongue. This is generally a better bet for purposes of communication, as long as you can trust your party’s interlocutors when they don’t share your alignment.)
All of this is important to keep in mind for DMs inclined to tweak the rules to match their ideas for their setting. If monsters don’t share a language with the player characters, negotiation becomes a lot more difficult. You’re more likely to see combat become the standard—indeed, the only—mode for resolving encounters, while the alliances with monster factions that are characteristic of Gygaxian play go by the wayside.