I am often contacted by foreigners visiting or living in the Tokyo/Yokohama area looking for ballet classes for adults. I am pleased that so many understand the the benefits and enjoyment to be had from dancing!
I don't teach any adult classes myself but I can suggest you look into the following studios:
Architanz (near Tamachi station on the JR Yamanote line, Tokyo) is an open studio that offers classes for adults. Many of the teachers are visiting from abroad so often the classes are conducted in English. Here is the English side of their website:
IAC is a studio in Shibuya that caters only to adult dance students. Basically they operate in Japanese but they have a full schedule of different dance classes and a dedicated following. Here is their website:
Chacott, Japan's biggest chain of dance supply stores, also runs a number of studios. Here is an English list of their stores and studios. You may have to contact them in Japanese, though, to get information:
Hope this helps! Happy dancing!