Good question, CancerMan. Here are some good choices:
Low Atlantean are the defacto human language of the setting, the equivalent of Common. Mostly civilized races and some monstrous races speak it, such was the influence of this fallen civilization.
High Atlantean is kind of like Latin in our world. It is mostly known by scholars but also by those a few who toil in ancient ruins for lost treasure. Magic and technological secrets are often enshrouded in cryptic High Atlantean.
Dwarven is rarely known outside of the halls of the Dwarf Lords, but with a good tie-in with your backstory it could make sense. Dwarves do trade with the other peoples of the world, and are the keepers of the secret of steel and other lost smithing techniques.
Elven is difficult to master for humans, requiring years of study. Few ever master it.
Urgan is the beastial language of Beastmen and their kin (Gnolls, Apemen). It is verbal only and has no written script. Those who war against or trade with these creatures sometimes learn this primitive language.
Draconic is the ancient language of dragons and is still used by the races that once served them: Serpentmen, Troglodytes, Kobolds, and some practitioners of fell magic. Although humans struggle to make the low, gutteral noises and reptilian hisses and clicks, it is conceivable to learn from extended interaction with such creatures.
More rarely, there is also
Benthic, the tongue of Aboleths and other Deep Ones
Giant, the tongue of Giants, Cyclopses, and Minotaurs
Rakshasan, the tongue of the Rakshasas
Abyssal, the tongue of the Hells
Celestial, the tongue of Gods, Angels, and sometimes priests