I found this!
Dhimmitude.orgAnd the Pact of Umar:
ARTICLES OF THE PACT OF UMAR
We Christians:
1 - We shall not build, in our cities or in their neighborhood, new monasteries,
2 - churches,
3 - convents,
4 - or monks' cells,
5 - nor shall we repair, by day or by night, such of them as fall in ruins
6 - or are situated in the quarters of the Muslims. . . .
7- We shall not give shelter in our churches or in our dwellings to any spy,
8 - nor hide him from the Muslims. We shall not teach the Quran to our children.
9 - We shall not manifest our religion publicly
10 - nor convert anyone to it.
11 - We shall not prevent any of our kin from entering Islam if they wish it.
12 - We shall show respect toward the Muslims, and
13 - we shall rise from our seats if they wish to sit.
14 - We shall not seek to resemble the Muslims by imitating any of their garments, the headgear, the turban, footwear, or the parting of the hair.
15 - We shall not speak as they do,
16 - nor shall we adopt their honorific names.
17 - We shall not mount on saddles,
18- nor shall we gird swords nor bear any kind of arms nor carry them on our persons.
19 - We shall not engrave Arabic inscriptions on our seals.
20 - We shall not sell fermented drinks. . . .
21 - We shall not display our crosses or our books in the roads or markets of the Muslims.
22 - We shall only use clappers in our churches very softly.
23 - We shall not raise our voices in our church services or in the presence of Muslims,
24 - nor shall we raise our voices when following our dead.
25 - We shall not show lights on any of the roads of the Muslims or in their markets.
26 - We shall not bury our dead near the Muslims.
27 - We shall not take slave who have been allotted to the Muslims.
28 - We shall not build houses over-topping the houses of the Muslims. . . .
(When I brought the letter to Umar, he added, "And we shall not strike a Muslim. We accept these conditions for ourselves and for the people of our community, and in return we receive safe-conduct.
Hrm. Fair enough. It would have been selectively enforced, you say? I can believe that; many laws are. Though I'm sure it would still impact the day-to-day interactions of muslims with the dhimma.
Back to the topic at hand: Shall we settle on a universal language? It's not implausible for either of the Western characters to know Latin - each is a devout Christian. I can justify it for Marduk - although he has no formal education, his master is well-educated and most likely uses Latin from time to time. He may have picked up enough to speak it.
I'm really not sure about Mordecai. But I've seen worse transgressions for the sake of facilitating an RP. xD.