You don't need electronic help to take the subway in Tokyo. Most stations have a system map in English somewhere, as well as information in English about what is located at each exit. Even if you can't find these aids to navigation, there are (gasp!) paper subway maps that are easy to obtain and carry around.
Earlier this summer, I was in Osaka, a city I don't know well, and I had to visit a long-time client who had moved from one part of the city to another. My hotel was able to provide a paper subway map in English, and the front desk clerk traced out the easiest route on it. (It still required two transfers
) Once I arrived at the station closest to my client's office, there was an exit sign with the name of the office building in English.
Even if you get lost in the Tokyo subway system, it is highly unlikely that anything bad will happen to you, and Japanese people are usually helpful to puzzled tourists. Furthermore, since it's Tokyo, with tens of thousands of foreign residents, you may well find a friendly expat who will help you out.