Level 10: A thief can read and cast magic from magic-user (and
elf) scrolls with 90% accuracy. A failed roll means the spell does
not function as expected, and can create a horrible effect at the
Labyrinth Lords discretion. (page 14, Labyrinth Lord, by Goblinod Games)
The Bard in D&D 3.5 was able to cast arcane Magic, so it appears that when developing the class for C&C, they decided to give it the opportunity to decipher much like the thief/rogue. The Bard, IIRC got introduced to the D&D community in the 80s tome Unearthed Arcana - which I dont have. But I would suspect it was something similar to what the Thief ability was if not accessible at an earlier level due to the semi-magical nature of Bards and their songs. In a LL supplement (3rd party) by Brave Halfling Publishing, the Bard Class is given thea ablity to Decipher Magical script at an increasing percentage base (starting at like a 17% chance at 1st Level and increasing).
Magic-User / Elf Scrolls As dabblers in the arcane arts,
bards may attempt to read and cast magic from magic-user
and elf scrolls. Just like the thief ability, a failed roll means
the spell does not function as expected, and can create a
horrible effect at the Labyrinth Lord's discretion. (page 3, Delving Deeper - Bard, by Brave Halfling Publishing)
I think the Trolls gave Bards & Rogues the "Decipher Script" ability as a nod back to the older editions of D&D for both the skill of reading languages and reading magical languages. The big thing to remember is that it is incredibly hard for a Bard or a Rogue to read magic script at early levels (hence the -10) though at 10th level that penalty would all but disappear. Further, if it wasn't a prime ablity, again, still very difficult to do even at higher levels.
yes, I know that may explain the history and background, but id hasn't answered your original question yet!