I will readily admit to rarely paying attention to language rules in D&D. Usually when the party wants to parlay I just make a snap decision (perhaps based upon a die throw) whether or not the creature speaks a language known to the adventurers. But recently I decided to take a look at the languages spoken by various critters in 1st edition AD&D. I was mainly curious as to the default answer to the question "Who can speak to whom?" or perhaps "Who can speak to what?" All my data here is from the Monster Manual. I'm going to break down the results of my survey into four broad groups.
Group 1 - PC Races
Dwarves speak dwarvish, gnomish, goblish, kobold, and orcish. While all dwarf PCs speak common, those encountered "as monsters" have only a 75% chance of knowing the common tongue.
Most elves speak goblish, orcish, hobgoblish, gnollish, common, elvish, halflingish, and gnomish. Wood elves speak only elvish, treantish, and "the languages of certain woods animals".
Halflings speak common, halflingish, gnomish, goblish, and orcish, with tallfellows knowing elvish and stouts speaking dwarvish. PHB halflings all speak elvish and dwarvish.
Gnomes speak gnomish, koboldish, goblish, halfingish, dwarvish, and with burrowing mammals.
(Aside: Some early versions of D&D do not specify any starting bonus languages for halflings, so that default halfling PCs don't speak their own language! In my campaign I assume that halfling is a dying language as the wee gits are suffering from slow cultural assimilation by humans.)
Group 2 - Humanoids
Kobolds speak koboldish and 75% speak goblish and orcish.
Goblins speak goblish, kobold, orcish, and hobgoblish
Orcs speak orcish, the majority of them also speak goblish, hobgoblish, ogrish.
Hobgoblins speak hobgoblish, goblish, orcish, and the language of carnivorous apes. 20% speak common.
Gnolls speak gnollish, trollish, and 60% know orcish and/or hobgoblish.
Bugbears speak bugbear, goblish, and hobgoblish.
Ogres speak ogrish, orcish, trollish, and stone giant.
Minotaurs speak their own language and 25% speak common.
Trolls speak only trollish.
Ogre Mages speak ogrish, ogre mage, and common.
Stone giants speak only stone giant.
Group 3 - Sylvan races
Brownies speak brownie, elvish, pixieish, halflingish, and spritish.
Pixies speak pixieish, spritish, and common.
Sprites speak spritish and common.
Sylphs speak sylphish and common.
Dryads speak dryad, elvish, pixieish, and with plants.
Nixies speak nixie and common.
Nymphs speak their own language and common.
Satyrs speak satyrish, common, and a form of elvish understandable only by wood elves.
Treants appear to speak only treantish.
Group 4 - Miscellaneous Monsters
Some dragons speak human languages.
Lamias speak common.
Leucrottas speak common.
Androsphinxes and gynosphinxes speak all sphinx tongues and common.
Criosphinxes speak their own language, andro/gynosphinxish, and with all animals.
Lycanthropes have their own languages, but speak common as well.
Titans speak their own language, all six giant tongues, and common.
Observations and Speculations
When dwarves and elves meet, they can communicate in gnomish, goblish, or koboldish. My guess is that gnomish is the polite choice, while the humanoid tongues are used to demonstrate disdain.
Gnomes can communicate with Criosphinxes in the tongues of burrowing mammals. Criosphinxes can communicate with Hobgoblins in Carnivorous Apish.
The number of sylvan folk who speak common surprised me. I'm led to consider the possibility that every human village knows one or two local fey.
A titan and an ogre meet at a crossroads. They start telling each other dirty jokes in stone giantish.
Speaking of ogres, for big dumb gronks they know like four times as many languages as I do!
The PCs need the help of the Treant King, so they must first gain the help of the local wood elves to act as interpreters. I hope a party member speaks elvish!
I did not know there was a wererat language.
If you want to parlay with the monsters orcish and goblish seem like good languages to know.
Given the ubiquity of orcish, its regular association with goblish, and the status of orcs in my campaign as goblins magically enhanced for combat, I've decided that orcish is in fact the monstrous equivalent of common. Basically orcish is a mish-mash patois, based upon a foundation of goblish but peppered with idiomatic phrases and vocabulary borrowed from hobgoblish, koboldish, ogrish, gnollish, trollish, stone giant, and even the gruntings of carnivorous apes. Aside from those listed above, about 1 in 6 monsters capable of speech know enough orcish to be able to ask where the bathrooms are located.
Why do the good guys speak so many humanoid languages? Why do so many dumb humanoids speak mutliple languages? My first thought is that despite all those stories about racial wars between dwarves and goblins (and etc.), they actually do a lot of trading during peacetime. Maybe it's only when some jerkface politician whips up racist sentiments that the dwarves remember they're 'supposed' to hate those guys. "The gold mine has gone dry and unemployment is up? Clearly the goblins are to blame!"