Came from Free "Logic" web games! Took a few hints for me to get it, but does Translate have an easier solution than the hint?
The hint takes into account whether the Vendor can lie, but Vendors can't lie, can they? Only guards can?
I just had the Vendor ask the guard if a door was a symbol, and whether the answer symbol was different from the asking symbol? Following the solution given in Language's Hint, a.k.a the hint from Puzzle 9, then the True / False answer corresponds to the door?