Re: Do I need Japanese to practice in Japan?
Until about a year ago I was living in Minami-Gyotoku--a few stops down the Tozai line from you. Memories!
Anyway, for me, trying to learn aikido without understanding Japanese was very frustrating. I walked into the Aikikai Hombu Dojo with no previous experience besides a few aikido-related conversations with my co-worker, thinking that with determination alone I could blow a hole through (or turn tenkan around) the language barrier. It's all physical, I thought--I'll just watch and imitate. Nope. On a good day, I heard a half-dozen English words from the instructor, and got five minutes with a partner whose English and patience was good enough to help me learn the technique. On a normal day, I trained with partners who gave me bored looks and took falls to avoid having to explain anything to me. On a bad day, I got paired up with 250-pound Europeans who certainly did not travel halfway around the world to train with skinny, clueless me--these people, after some non-English growling, would helpfully reverse my non-existent techniques to demonstrate just how clueless I was.
So, after three months of training four days a week, I still had little understanding of the principles behind the techniques, which meant little idea of when I was doing something right or wrong, which meant little sense of progress. One day, the instructor came to me, corrected my stance, laughed, muttered, "Basic..." and walked away. I decided that it was ridiculous to wait three months for five seconds of instruction on how to stand (which I may or may not have understood), and I never went back.
Instead, I typed "aikido english tokyo" into Google and found the Yoshinkan Hombu Dojo (conveniently located on the end of the Tozai line.) They have an English class in the mornings, with patient, friendly instructors (who, fittingly, gave me plenty, plenty of instruction on how to stand.) I went there for the last six months I was in Japan, and almost always left the dojo in a good mood.
Now that I've spent almost a year practicing aikido in the States, with an instructor who explains the intricacies of techniques, I would never recommend starting aikido with a teacher who doesn't speak your language. Aikido's just too subtle. Even now, I'm not sure how much I would get out of going back to Aikikai Hombu besides practicing things I already know. I'm sure things might be better for you if you can find a smaller dojo (my Aikikai classes normally had twenty to thirty people), but I still feel that it would be more efficient for you to either learn Japanese first or find an English-speaking instructor.
In any case, good luck!