The Yasna is written in two layers of one language. The Gathas and the closely related Yasna Haptanghaiti survive in Old, or Gathic, Avestan, an archaic form close enough to Vedic Sanskrit that comparative philologists have used the two to reconstruct features of each. The surrounding chapters are in Younger Avestan, a later stage of the same language family, which is how scholars separate the oldest core from the liturgy built up around it.
Avestan had no dedicated script for most of its history. The material was carried orally by priests for centuries, memorized rather than read, and the alphabet now used to write it was devised only under the Sasanian Empire, broadly in the period from the third to the seventh century of the common era, specifically to fix the pronunciation of a ritual language that had drifted far from any living speech. Manuscripts of the Yasna that survive today descend from that written standardization rather than from any earlier text object, so a fixed date for when the Yasna reached its current chapter order cannot be stated with confidence.
Alongside the Avestan text, later tradition transmits a Zand, a translation and commentary in Middle Persian, that Zoroastrian priests used to teach the Yasna's meaning once Avestan itself was no longer understood as a spoken language.