If you just keep to the languages in the PHB, it almost never matters. Almost everyone will speak common, even a good chunk of monsters. In my Greyhawk, I've included the ethnic human languages and ancient versions of many racial tongues to add variety. All human speak their ethnic language, and while the majority speak Common as a 2nd language, not everyone will. While it's generally considered rude to speak anything but Common among outsiders, most people are going to speak their local language when casual. This greatly shuts down eavesdropping, unless they happen to also speak the same language. I've also modified the number of languages based on Int modifier, including a negative reducing your number of languages. All of this makes potential language barriers an issue, improving the value of Comprehend Languages and Tongues.