The following are some of the languages spoken in the world of Greyhawk. They can be learned with the Speak Language skill, though exceptions may apply where noted. Unless your character speaks a particular language, assume that knowledge or recognition of it is OOC information.
Khen-zai: the racial language of khen-zai, or ethergaunts, a spindly race of alien humanoids created by powerful aberrations in the distant past, that now dwell in the Ethereal Plane. They cannot speak, but use special tendrils on their head to produce a psychic sensation of sound in the minds of their own kind, or of those they have psionically dominated. Khen-zai is the language they use for this, but it cannot be understood or learned by other sane races.
Kuo-Toan: the racial language of the kuo-toa, an evil race of froglike monstrous humanoids. It contains a lot of croaking sounds, which can be hard on the throats of other races. Drow sometimes know it, as they often trade with nearby kuo-toa settlements. Rare outside of kuo-toa settlements, though resources on it can be found in larger towns and cities.
Leskylor: the racial language of the leskylor, a race of winged tigers native to Eronia, the second layer of Elysium. Despite their fearsome appearance, they are highly intelligent and kind, and have a complex language of their own. Very rare outside of Eronia, though resources on it can be found in larger cities.
Loxo: the racial language of loxo, a race of towering, nomadic elephantfolk found in places like the Gnoll Lands. Loxo has many deep and guttural sounds. Its pictographic written form is rarely used, as the nomadic loxo must travel with few possessions, but is often found carved into the living tusks of storied tribe members as a physical display of their achievements. When a loxo dies, its tusks are removed and used as grave markers, except those that are completely covered with carvings--seen as a mark of a life well-lived, these are carried by the family as holy heirlooms, to help teach the tales of their ancestors to future generations. Rare outside of loxo communities, which tend to be found in warm plains, though resources on it can be found in larger cities.