Well, for some styles of play, a hell of a lot.
Currently I'm running a fifth edition Forgotten Realms game. It's the first fifth edition game I've ever run. It's the first game I've ever run in the Forgotten Realms.
The Forgotten Realms language system is a mess. It's a complete cluster of weird related languages. For example. Illuskan is the language of the north spoken by the Illuskan people. But that's not all. There's Bothii, the language of the northern barbarian tribes. Uluik, the language of the native icehunters. Reghedjic, the tongue of the northern glacial Barbarians in the north and east.
Several people in the party only share the common tongue, which I have prepared a primitive word list, only allowing those words to be communicated in common. Three people speak Illuskan. Three speak Chondathan. Only one person speaks both.
This is awesome. It isn't a problem or something to be ignored. Communication and the trials associated with it are awesome.
Dwarven, That's the name of the language, right?
This is silly though, right? I've always found the third edition language system to be a bit absurd.
I speak gnoll, orc, dwarf and elf!
It's simple, straightforward, and bland. The core rules of 5th edition keep it this way, but they have vastly improved the means by which you acquire new languages (covered in the downtime section of the core rulebook). This system is great for not needing to worry about language.
Something a little more interesting?
In most of my settings. I do something a little different, though quite similar in concept. Instead of breaking the languages up by species, I group them according to region and philosophical approach to the world. An example of my traditional language list:
Common (trade language), Human Regional languages (Varies) Fay, Dialects→High/Grey/Wild (Pixie-Farie, Elf, Grey), Lilliputian (Gnome, Halfling, Burrowing mammals), Hellion (Goblinoid, orc), Montus (Dwarven, Undercommon), Canis (Gnoll/Gnarl-ron/Wolf-men), Homonid (Ape), Jotnar (Trolls/giants), Suidae (Boar-men), Ophidian (reptile/lizard-men), Arachnidia (spider), Chordata (Kuo-Toa, Troglodites, Bullywugs), Sign, Black Speech, High Ancient
Restricting characters to very few languages (as fifth edition does) and making the selections meaningful is a good way to go.
The Beauty in Complexity
The fantastic thing about what's going on with the Forgotten Realms is that it's a hot mess. This is actually a lot like the real world. Most places you're going to be able to speak pidgin english, but each region and culture will have it's own language. This little bit of extra detail and time spent thinking about it makes play seem all that much more meaningful.
The Half-Orc in the party keeps saying these wise and philosophical things, but nobody can understand a word. The thief can barely communicate with anyone. The wizard can cast comprehend languages, but can't talk back to anything that's talking to the party, and half the time they are using him to talk back and forth to each other.
It makes for a good time.
A Final Note
It's important to remember this has a lot to do with your style of play. An open campaign where characters really exist and live in the world? It's great! Messing around with languages in megadungeon play will likely only get in the way of the actual gameplay.