[an unfinished sandbox]
Language pages by World:
Languages of Oerth
Languages of Toril and Abeir
Languages of Athas
Languagse of Krynn
Languages of Nerath
Game Glossaries:
character classes
races
weapons and gear
spells
monsters
key phrases for combat and social/parley
For Real World-based languages, see foreign translations of D&D:
Portuguese (for Mystara's Verdan language):
Humanos • Elfos • Meio-elfos • Gnomos • Anões • Meio-orcs • Metadílios
Bárbaro • Bardo • Clérigo • Druida • Feiticeiro • Guerreiro • Ladino • Mago • Monge • Paladino • Ranger
Spanish (for Mystara's Espan language and Flamenco dialect of Elvish):
French (for Mystara's Sylaire language)
German (for Mystara's Hattian dialect of Thyatian Common)
etc.
DM Resources:
Google Translation
cheap Hippocrene dictionaries
Fantasy Linguistics:
"Gamer's Pidgin" (Using foreign languages in a game session, such as a greeting from an NPC or a sign or scroll in another language; Doesn't have to be grammatically correct. Just use Google Translate, or grab a dictionary and use the words to make a sentence in the same way as English, without worrying about conjugation or grammar.)
Rationalizing the existance of a Planar Common Tongue in the Second Edition Reality of the Planescape setting.
Elvish, Dwarvish, Draconic, etc. the same in all D&D worlds (except for cultures with special flavors, such as Flamenco Elves of Mystara)
Language can be a mixture of two or more Real World languages. E.g. languages in Ravenloft.
Language of Translation ("LOT"):
Common isn't really English, it's "translated" by TSR/WotC into English. Foreign-language editions are translated into Spanish, etc. For example, the City of Greyhawk is Falcongris, Faucongris in the Spanish and French editions. This indicates that the name isn't really pronounced g-r-ay-h-aw-k in Oerth Common, but simply means "grey hawk".
However some languages in some settings are really "English". For example, the language of catfolk of Bellayne in the Red Steel setting is really English, since Bellayne is supposed to be a fantasy-version of Britain.
This only applies to Common. Just because a language is identical to, or evocative of, a Real World language, doesn't mean it's fictively translated.
Fictive Translation within a Fictive Frame:
Idea that all D&D books are translated by TSR/WotC employees from actual documents from beyond the Plane of Shadow. Like Tolkien's Red Book of Westmarch.
"Local Flavor" concept for making flavorful "dialects" but keeping the number of languages low:
"Dialects" can be just the usual D&D Elvish, D&D Common, etc., but with a distinct set of proper names (personal names and place names), and local flavor-words. Basically, the DM throws
Elvish (Belcadiz) = D&D Elvish + Spanish proper names and flavorful words
Thyatian (Hattian) = Language of Translation, such as English + German proper names and flavorful words
Thyatian (Darokinian) = L.O.T. + generic Anglo-Celtic fantasy names
"Generic Anglo-Celtic Names", a Fantasy Trope
Linguistic Studies:
Mystaran weekdays and months reveals actual Common language.
Gather list of all languages in all editions of D&D
Gather all names from D&D products
Gather all extant wordlists from D&D products, Dragon articles, etc. (bree yark! = Goblin "hey rube!"; keep for Orkish in 3E and Giant in 4E)
Have David Salo or Mark Okrand make actual D&D languages.
Wordlists, Dictionaries, and Grammars.
Language changes across rules editions:
Earlier editions had many more monster languages.
They still exist in those Realities (e.g. First Edition Reality, Second Edition Reality).
But "Reverse-engineer" them to be predictably similar to the languages they "merged" with in the Third Edition Reality and Fourth Edition Reality. E.g. Dryad language in the 1e and 2e Realities turns out to be similar to the other monster languages which became Sylvan in 3e. In 1e and 2e Realities, they are considered separate languages, but in 3e and 4e, they are a single language (perhaps keeping "Flavor Words").