Why was Araragi unable to understand Dramaturgie / Episode / Guillotine Cutter when they first met?
He was speaking another language.

Going off the English version of the Kizumonogatari novel, page 81 shows that Dramaturgy spoke something that Araragi didn't understand. Vertical displayed this by just having a bunch of black boxes in quotations (dunno how Nisioisin did it in the Japanese novel, but I'm sure it's something similar). Page 82 starts with Guillotine Cutter reprimanding Dramaturgy for not using "the language of the land you're in", meaning that Dramaturgy was speaking a language other than Japanese.