My guess would be Draconic, and for specific spells: Infernal, Celestial and/or Primordial. Is there an official list of which languages are often found on Spell Scrolls in 5e?
Spell scrolls are not written in a given language:
A spell scroll bears the words of a single spell, written in a mystical cipher. If the spell is on your class's spell list you can use an action to read the scroll [..] Otherwise, the scroll is unintelligible. (DMG 200, emphasis mine)
The DM can decide that it uses the runes of or resembles a language if he wants to add flavor or provide a clue, but knowing the language will not help in reading the scroll.