Another big info-dump post. Here's a list I compiled of all the languages mentioned in the World of Greyhawk set and the various AD&D monster books (where creatures are mentioned as "having their own language"). The initial idea behind this was to come up with a more comprehensive version of the Random Language Determination table on DMG p. 102 but there ended up being so many languages to render that impractical (at least for the moment). So instead of a table I'm just presenting it as a raw list.
Baklunish (spoken in Ekbir, Ket (alongside Common), Tiger Nomads, Tusmit, Ull, Wolf Nomads, and Zeif)
Flan (spoken in Geoff (alongside Common), Rovers of the Barrens, and Tenh)
Old Oeridian (spoken in Great Kingdom (including Medegia, North Province, Rel Astra, and South Province) and Ratik, generally alongside Common)
Rhennee (spoken by the Rhennee people, alongside Common)
Human Regional Dialects:
Fruz (Suloise/Flan dialect spoken by the Frost, Ice, and Snow Barbarians and in Stonefist - 40% compatible with Suloise and Flan)
Keolandish (Oeridian dialect spoken in Bissel, Gran March, Keoland, Sea Princes, the Ulek States, and the Yeomanry, often alongside Common - 60% compatible with Oeridian and Common)
Lendorian (Suloise dialect spoken alongside Common in the Spindrift Isles - 60% compatible with Suloise and Common)
Nyrondese (Oeridian dialect spoken by peasants and shopkeepers in Almor and Nyrond (alongside Common for learned people) - 60% compatible with Oeridian and Common)
Velondi (Oeridian dialect spoken by rural folk in Furyondy, Veluna, and Verbobonc - 60% compatible with Oeridian)
Archaic Human Languages:
Ancient Baklunish (ancient version of Baklunish still spoken in Plains of the Paynims (alongside Common for traders and educated folk) - 60% compatible with modern Baklunish)
Suloise (dead language now read only by scholars)
Human Foreign Languages:
Common Non-human Languages:
Uncommon Non-human Languages:
Merrow (dialect of Ogrish)
Rare Non-human Languages:
Carnivorous Ape (rudimentary language)
Atomie (dialect of Sprite)
Pan Lung/Shen Lung
Very Rare Non-human Languages:
Greenhag (dialect of Annis)
Svirfneblin (dialect of Gnome - 60% compatible)
Common Tongue of Hades
Alignment Languages (nine in total)
Ferral (Oeridian dialect now used as a secret code language among officials of the Iron League - 60% compatible with Oeridian)
Subterranean Trade Language (“Undercommon”)
I drew a distinction between Ancient Baklunish (as described in the WOG Guide p. 16) and modern Baklunish (per the table on the WOG Glossography p. 31).
The "Human Foreign Languages" were all made up by me, based on various off-map lands mentioned in Gary Gygax's Gord novels.
The "common" non-human languages are the nine listed on p. 34 of the Players Handbook plus the three additional languages (bugbear, gnome, hill giant) that have a 2% or higher occurrence on the DMG p. 102 table, which seemed like a reasonable standard. The "uncommon," "rare," and "very rare" lists are based on the monsters' Frequency (with common monsters that don't fit the above criteria included on the uncommon list).
Non-human languages with asterisks are those included in the table on DMG p. 102.
Non-human languages in parentheses are not mentioned in the books but I'm assuming based on the nature of the creatures that they probably have their own language (and note that the DMG p. 102 table includes several monster-languages that aren't mentioned in the Monster Manual: ettin, gargoyle, manticore, naga, salamander, and xorn).
The other-planar languages are mentioned in the books and are not specific to one monster; based on these it can probably be extrapolated that each Outer Plane has its own Common language (that presumably, like the modron language as described on MM2 p. 86, is related to that plane's corresponding Alignment Language(s)).
Various monster descriptions mention the ability to speak with types of animals - burrowing mammals, woodland animals, snakes, birds, fish, etc. I chose not include any of these as languages per se.