A quick piece of advice here for DMs. When your players are generating their characters, make sure they remember to write down which languages they speak – in my experience, it’s commonly overlooked. While you can often write other things in later, you generally only notice that you’ve forgotten to include languages when asked if you speak a certain language. Almost always, a player in this position will have to decide whether or not to write that language in on the spot to fill in their character sheet – in which case, even though it’s probably the right thing to do and isn’t that big of an advantage later, it may feel like cheating.
Most people in English-speaking countries will rarely encounter other languages in their daily lives, and so it’s easy to overlook this aspect of a game where learning new languages is remarkably easy for a heroic character. Writers fail to take advantage of this aspect too, so we end up with worlds where everyone speaks Common. I’ve heard homebrew settings discussed where there is no such thing as a common tongue, which could be interesting if you’re willing to take that route.
Even with Common in place, consider situations where knowing other languages has an advantage. Dwarves place great importance on tradition, so speaking to an old dwarf in his native tongue might help to gain his favour. Old documents might be written in another language and you might not want the wizard to waste a spell slot on comprehend languages when he could prepare an offensive spell instead. You can overhear opponents who don’t expect invaders to know their language. Perhaps you can all speak one language in common and your opponents won’t understand you.