Languages associated with spellcasting and spell craft. Draconic, infernal/abyssal/celestial, giant, some sort of ancient language. This gives you an idea.
1e didn't have any specifically "spellcasting-associated" languages. However, you could learn the language of certain intelligent monsters; a partial list is below:
Beholder, Brownie, any single color of Dragon, Dryad, Elf, any single type of Giant (esp. Cloud or Storm Giant), Gnome, Lammasu, Mind Flayer, Naga (Guardian, Spirit, or Water), Nixie, Nymph, Ogre Mage, Pixie, Shedu, Sprite, Sylph, Titan, Androsphinx, Ixitxachitl, Ki-Rin...
Also, I'm throwing in Ancient versions of a lot of languages. Just call it "Ancient Common" or "Ancient Elf" or whatever.
Ultimately though, since in 1e, humans were the only race that could become the greatest wizards ever, the most important language for spellcasting documents... is Common.