After a discussion about languages known by PCs and their relevance/irrelavance I decided to look at the languages in the world I run (Faerun), the world I live in and how many languages PC probably should have and how DMs should? Be using languages. I live in the US and after taking varying amounts of 5 languages at one time or another i am a monolinguist. Here in the US I suspect that the majority of non 1st/2nd generation immigrants are also monolinguists (English) and we Americans sort of take this for granted. By the same token I’m aware that this not the same in other parts of the world. In Europe’s much (most?) of the population is multilingual. The same in many parts of Asia. When I lived in Malaysia it was not uncommon for someone to speak English, Chinese, native Malay and possibly Arabic. I just got through looking at the languages for Faerun and in essentially every region multiple languages are spoken besides “Common”. I suspect that Faerun is much like Malaysia where most folks are multilingual. Given that the present language rules in 5.24 typically grant common + 1 other language (humans get 2) so that different species can take (only) their species language without taking the linguist feat (+3 languages) Ranger’s get 2 more at L2 and the upcoming banneret will get some extras as well. Given what I’ve seen and experienced this seems to be an American centric understanding of languages and language knowledges. I know I tend to run most things in game on just common and suspect I’m doing my players a disservice. The closest I’ve really come are having rangers ( or others) listen in on humanoids etc if they have those languages. That or once a PC had taken Bothii (Urthgardt barbarian language in Faerun)  and was able to use it to speak to an Urthgardt party and persuade them we were friends. So I’m wondering how others use languages in their worlds and whether PCs should maybe have more languages so we can really use languages in game.

I have run campaigns where language mattered in the past.  From a world building perspective, I thought it would make the world richer and more flavorful….

I was not fond of how it actually played.  In my experience, heavy reliance on language proficiency typically resulted in a singular player having a bit of a main character vibe whenever the party reached a social encounter, even if that character was not suited for that type of encounter of the player did not like being put on the spot like that.

Now, I use it mostly for a bit of flavor.  You can choose to interact with people in their native tongues, and that might give you some various boons (they might be more friendly to you, for example, or you might overhear something they didn’t think you could understand), but it is not a necessity.

Language proficiencies I find a bit more also helpful when implicated in puzzles, dungeons, or when finding writings.  Since these are more one-off “you read the thing” situations, instead of protracted dialogues, they tend to be a fun reward for the proficiency, without resulting in other players twiddling their thumbs for an entire conversation.

All told, I firmly put language proficiencies in the “fun to have, fun to sometimes use, but they create problematic gameplay when elevated front and center.” 

One detail I've noticed that feels sort of backwards is that proficiency with normal languages like dwarvish, elvish, halfling, and gnomish very rarely makes any difference. Unusual languages like primordial, on the other hand, do come up every now and again.

Quote from Frachese >>

One detail I've noticed that feels sort of backwards is that proficiency with normal languages like dwarvish, elvish, halfling, and gnomish very rarely makes any difference. Unusual languages like primordial, on the other hand, do come up every now and again.

 
This is a table by table thing.  My table uses languages a lot so my players prefer to get a wide variety of them.

I generally try to avoid it. In my current game, there's a specific deity (whose portfolio included traders and merchants), who deliberately promulgates the common tongue, so almost everyone speaks it. Similarly, the species-specific pantheons maintain their own universal languages (including human). If I were doing it again, I'd probably make it so that using social skills in Common gives disadvantage, which would still avoid communication breakdowns (they're always the same), while still making language proficiencies mean something.

The other thing is that Children mp. lang. boy lets you understand the language not speak it. So it’s great for reading ancient dead languages or unknown languages but not great for communication. For that you need it and tongues which is much higher level. If you’re going to make languages a major part then tongues and comp. Lang. Need to be much closer in level.

To be honest, I've been put off of the concept of using languages much by my experiences as a player. It's come up twice, and both times my experience  was diminished by not having the "right" language. The first was playing Tales from the Yawning Portal and because I did have the right language, I was locked out of part of the quest. The second time was a one shot where I was playing a Changeling. I decided that upon approaching the the bandits lair, I'd lure the guards out to be ambushed. I'm not sure if the guards were retconned or if they always were that kind and I was just unlucky,.but the DM decided because I didn't speak their language, I couldn't do it and I got piled on. That being the entire conceit behind my character kind of ruined the session. I mean, I still enjoyed it overall, but it was much less fun with the core of identity of my character shut down.

Neither left me feeling like my experience was better for the presence of languages.

I'd be open to puzzles based around it. "Here's a language you don't recognise, but here are some clues on how to translate the words so you can figure out how to progress" has potential to be fun. I'm not keen on the idea of locking things behind languages so if you don't pick the "right" ones, you can't complete certain parts of the game or just waste those choices.