- How important is it to speak German?
Not that important for the event in general. Many games either don't require language to play or are available in English. Staff will be able to explain game rules in English.
However, I think most of the RPG sessions will be held in German. It's possible that there are some in English, but I'm not sure.
- How hard is it to get a hotel in the immediate area? How long in advance should I book? Do you book directly or is there a housing board (like GenCon)?
No idea. Essen is a reasonably large city and it's part of the Ruhr metropolitan area. Public transport in the area is very good, I don't think it's required to stay within walking distance. If in doubt, ask the hotel how long it would take to get to the venue by public transport before you book.
- How does registration work? I assume you need a con badge but do you also need to register for individual games/events?
I've only ever bought tickets at the gate. There used to be one-day and multi-day passes, but I'm sure you can find out on their website.
Playing board games generally doesn't require registration. If you want to play one of the very popular games you may have to wait a bit for a set or table to free up occasionally. Consider sitting on the floor if table space is limited.
I attended an RPG session a few years back and they had a sheet where you could write in your name before the session as long as there were spaces left. Don't know if it's still done the same way.
- Is there anything else that would be helpful for a first time attendee to know?
If you're going with a group of friends, consider splitting up into groups of two or three and join other people to play. It will be easier to find two empty seats at a table than a whole table to yourself. If you see an empty seat at a booth you're interested in, just ask nicely and most people will invite you to join them.