Language is an ever-evolving thing in Golarion, especially since it draws on so much culture and history. The Inner Sea in particular has a number of older languages that form the basis for its modern languages or were widespread at their time. Here’s the information I found on these ancient languages.

Ancient Osiriani: This precursor to modern Osiriani shares many similarities and differs mainly in its hieroglyphics and lexicon. It seems anyone that speaks Ancient Osiriani can communicate with modern speakers, although with some difficulty. You can find Ancient Osiriani in the Golden Road region, particularly in Osirion of course.

Azlanti: Azlanti was probably the dominant language of its time and formed the basis for dozens of languages all over Avistan and Garund. Remnants of it remain to this day and countless scholars keep the language alive in the Saga Lands and among the Ruins of Azlant.

Cyclops: This language seems to have been widespread before the rise of humanity. The language of the cyclopes of Ghol-Gan seems to be the basis for modern Jotun, though Jotun incorporates a number of Thassilonian roots as well. A few speakers of Cyclops reside within the Shackles.

Jistka: The language of the Jistkan Imperium shares many words with Ancient Osiriani, though with different definitions, which possibly contributed to their conflict. Scholars and royalty seem to be the majority of Jistka speakers in modern day. These speakers are concentrated in the Golden Road region but are scattered throughout Avistan.

Orvian: This language hails from the furthest depths of the Darklands and is likely still in use in many remote parts of the region. Significant portions of Orvian became part of modern Undercommon.

Tekritanin: The language of the Tekritanin League was made up of many regional dialects. The origin of some of these dialects have been seemingly lost to time, creating a nigh-unsolvable mystery for modern linguists.

Thassilonian: Thassilonian formed a part of the basis for the modern Varisian language. It makes use of three runic alphabets that are returning to prominence thanks to efforts in New Thassilon and the greater Saga Lands.