I'm not sure on the final implementation of quests since the ones we had in beta were placeholder, but monsters do respawn. First, there are monster "lairs" that continually spit out a number of defending monsters until the lair is destroyed. Secondly, monsters just pop out in appropriate places. Large forests will spawn wolves and bears here and there, etc. That said, the majority of the content in the game will be one-off. Once you visit it, it's gone unless something happens to trigger another spawn (like quest techs trigger spawning of new quest locations).

It is confirmed. You would be able to do a mod translation but it would take some effort.

As you go through the quest tech tree more advanced quests become avalible and appear on the map, so yeah quest change.

Also consider that this game isn't just about exploring and questing.  There is a definite end-game goal of meeting one of the four victory conditions, and you'll be battling against opponents with the same goal, so you won't want to dawdle.

not that im terribly concerned, but is there perhaps a "quest mode" that doesnt take into account an endgame? just aimlessly building cities, questing, etc.?

Honestly, I'm not sure.  I think the single player campaign will be more about questing than conquering the world while the "sandbox" game will be conquest oriented.

Indeed. You can summon fantastic creatures via spells and build city improvements near ogre camps and the likes to train them. In addition, the more you advance in the diplomacy tree the more wild creatures that are roaming the world can be tamed.

They stay definitely until they are killed or you release them. I am sure that there are spells to summon creatures during combat, too. I think it will be very similar to Master of Magic, though I don't know how the upkeep mechanic works. One of the spell books is just about summoning stuff.

Unless it changed, your Essence score determines how many enchantments/summons you can maintain. So if you have, say, 5 Essence you can have 4 permanent enchantments and 1 summon, 2 enchantments and 3 summons, so on so forth.

Well, you can choose your victory conditions in sandbox mode - I'm not sure if one could turn off victory conditions altogether, but you could turn off things you're not interested in like victory by conquest or diplomacy.