I liked Gary's bit in the 1st edition DMG on alignment languages as limited tools that only cover relevant concepts or brief functional conversations. It made sense but still wasn't what I wanted. Then I remembered A MOST THOROUGHLY PERNICIOUS PAMPHLET. Those language rules were pretty close, but not quite what I wanted for my setting where the PCs explore a lost wilderness completely different from their home civilization.
So with all credit & apologies due to Mr. Torres, here is my try:
Languages that are related to one your PC knows give you a bonus to understand them: roll your Linguistics with a +5 bonus, and you will always understand at least a word or two even if you bomb the roll terribly.
IMPERIAL COMMON - Speech codified in the Empire around 250 years ago, all the PCs know it. Nothing exciting.
Descended from OLD HIGH IMPERIAL - the ecclesiastical language spoken and written by all the lost faiths outlawed and replaced by the Imperial 137 gods. Many old ruins in the wilderness have writings in this language. There are also other human languages descended from Old High Imperial that might be encountered in the wilderness.
WILD ELVISH - Spoken by feral elves & half-elves, who are all stone- or iron-age tribesmen, second-class citizens of the Empire living on reserves or in the roughest wilderness.
Descended from TRUE ELVISH - Spoken by the entire Elven race before its fall and division into warring sub-races. Ancient ruins, lost knowledge, spellbooks and scrolls will have True Elvish writing. Other descendants of True Elvish are DROW and BOG ELVISH.
OREMOUTH - Spoken by dwarves, gnomes and certain lesser elemental spirits. Often these spirits are bound into constructs, so golems, gargoyles, and animated statues can speak, or at least understand it. The higher elemental beings (water, air and fire) don't have their own languages.
LIZARDFOLK - Spoken by these swamp-dwelling primitives, it doesn't have a written component.
GRIMSCRIBE - The secret language of wizards, it is written only and has no spoken dimension. Not the same as the magic words they write down their spells with, it's just for communication between wizards - not sorcerors, witches or anything else. Demons who deal with wizards all know it, and contracts are often written in Grimscribe.
Both are descended from DRACONIC - Lizardfolk who are literate read & write in Draconic characters, but rarely can speak it. It's almost unseen as dragons are quite rare, but occasionally a magic item, book or legendary weapon will have Draconic writing.
SKELETONGUE - spoken by thieves, rogues, criminals and young or lesser undead. All over the Empire criminals have been executed & buried without funeral rites. So many thieves & bandits rose from their graves, they brought their secret speech from this life to the next.
Intelligent greater undead will speak whatever languages they did in life, depending on their origins.
WILD SPEECH - The high ritual pagan language of Neutral human tribes, before the Empire subjugated them and brought them 'up to date', outlawing their nature gods or incorporating them into the offical pantheon. Spoken by druids, rangers and some other humans who yearn for the old ways.
Related to the languages and dialects spoken by the wild tribes of the wastes. They don't have writing and their speech is much less sophisticated.
Both are descended from SKINCHANGER - the high language of all lycanthropes.
GOBBO - Spoken by goblins, hobgoblins and similar creatures the world over. They travel through the caves & dungeons under the earth and get absolutely everywhere, so PCs can start with this monster language if they want, and goblinoids in the wastes will speak almost exactly the same. There are dialects, but they amount to accents and regional slang.
HISSING - The language of the serpent-men of old. When they carved their mysterious monoliths and standing stones they used these hieroglyphics. Almost nobody knows it anymore, although nagas, yuan-ti, boa constructors and other beings speak dialects of it amongst themselves.
Descended from ELD - the language of all intelligent aberrations like aboleths, Deep Ones, etc. Even more rare, you would have to learn it from one of their servants, and it's probably better you don't meet them.
There will be no Celestial, Abyssal, Infernal or nonsense like that. Beings of extremely high or low spiritual vibration, described by mortals as 'angels' and 'demons' can either speak telepathically, automatically learn your language, or don't/can't communicate with squishy humankind in a way you'll understand.
Also, thanks to my buddy 4th for a good back & forth on the topic and giving me that crucial bit about Skeletongue I was missing to get things going! He said it best:
"Warcraft, Tolkein, Games Workshop and DnD have just confused the whole orc situation honestly. Like orcs are fine and hobgoblins are fine but do you think it ever happens that an orcish marauder has a fight with a hobgoblin slaver and they both stop and just wonder how it all came to this?"
While you're pondering these absurd new rules, jam this one next time you have a good GOBBO-BEATING in your game: