Word to the wise: this board has been acting wonky the past three or four days.
If you are making a lengthy post, I recommend that once you are finished typing it, you copy the text. That way, if the board fails to load after you hit "Submit" you can try again rather than losing it (which has happened to me twice now

).
Now, on to the questions. My responses will be in
blue:
1. Which other languages are available for learning besides the obvious racial ones (elven, dwarven)?
Lingua is the common language of the Aerdian Empire and is spoken throughout the empire and its (former) provinces, much like Latin was spoken throughout dark-ages Europe. Each province has subtle dialects, but for the most part anyone that speaks one can make themself understood anywhere in the empire and the surrounding area. Outside the empire, it gets a bit more diverse, but Lingua is quite widespread and it should be possible to make oneself understood (imagine speaking Italian in Spain - not the same, but similar enough to get by).
2. What are the common humanoids (goblin, orc, etc.) of the setting, if any, and what do they speak? I mean, this campaign may not have orcs (Krynn lacks them).
Orcs, goblins and hobgoblins are closely related and are common outside the empire. Within the empire the humanoid races were hunted pretty much to extinction centuries ago, although with the current troubles, it is possible that bands of them have managed to slip across the border from then north. The humanoids tend to speak a variety of polyglot languages barely understandable outside their own tribes. The basics can be called "Uruk" and speaking it can make you more or less understood when speaking to orc-types.
3. Are there any ancient civilizations before the Aerdian Empire, especially ones that spoke a strange language?
You can read about the ancient history of Greyhawk here. Don't go too far past that, though. This is a non-canon game.
In the area that is now the empire, the Flan people were conquered/displaced by the Oeridians, who established the empire many centuries ago. The Flan were tribesmen for the most part, and not kingdom builders, but shared a common language which is now all but dead. If you want a dead language on your sheet, call it "Early Flan"
Frivolous, what type of Cleric are you playing? I may end up changing to a different class if you're going to have the healing for the group covered....
I don't think he will. He's not that kind of priest.
Some more questions:
1. May I also ask if we are allowed to have hirelings or own slaves to take care of our baggage?
In the first adventure I have planned, the party will be traveling with/escorting a wagon, and can put small amounts of personal baggage on there, or they can use pack horses/mules.
I would recommend that the party have at least a couple of mules to carry food and equipment, and a hireling to manage the mules, feed them, etc. Maybe even a wagon.
Asking about slaves brings up an interesting point. Slavery is an accepted part of imperial society, although of late there has been a tendency, particularly in the provinces, to treat slaves like actual people. How an individual PC feels about slavery is up to the player. PCs will not start out owning slaves, however.
2. I know this is a low-magic setting, so I understand if it is not allowed, but can we have one or two objects with Continual Light on them, so we don't have to mess around with torches or lanterns for illumination?
Any priest over fifth level can cast Continual Light spells once per day or more, and could mass-produce continual light bulbs. But they do not. Spells are about doing their god's will in the world, not about making tools for adventurers.
The average village priest is too low-level to cast Continual Light. High-level priests are most often found in major cities or adventuring in war-torn areas.
So, if you want a Continual Light bulb, you can go to a city or war-torn area, find a priest who has one and buy, beg, borrow or steal it, or ask said priest to make one for you.
3. Can all of us start out as citizens? I'm presuming we want to start out as citizens, though some concepts may make that undesirable.
Whether a PC is dependent upon that PC's backstory.
4: Do spyglasses exist, and can we have one? They are not magical, but the PHB says they're very expensive, 1000 gold.
Yes, they do exist and they are expensive - too expensive to start out with, but you might buy one later if you have the cash.
What about Greek fire, for when we want to commit some arson?

Does Greek fire exist and can we have some?
Yes, it exists. Yes, you can buy it.