I finished D2:ED at level 36; without using the couple respawn points I think the highest I saw was level 37. In DKS with the changes in balance leveling was at least faster in the beginning, but I don't know if there was much of a difference later in the game.
Depending on whether you've been back to Broken Valley, I'd guess you are roughly 75-85% through the D2:ED section. Aleroth is fairly small (at least what you can get to before entering the Hall of Echoes), though there are a couple good traders there.
You can reduce mana costs, but not the recharge time. If you learn Magic Missile or Magic Blast you can alternate with Fireball.
There is no official way to change languages. I don't know if there is an unofficial way (you could change the German release to English text, but in that case both languages were included in the game).
