Hi, I have never lived in a foreign country, but I did recently visit Japan. Although I did not bring my son with me I thought I might be able to give you a little insight. I was in Osaka for 5 days. The people there are very friendly, and although there are not many that speak english, I found that they were eager to help the best they could. I learned a few key words like "hello", "bathroom" and "thank you". I carried around a cheat sheet in order to ask for specific things. If I needed directions people would always draw me a map. I did hear that if you really want to learn some Japanese that you should start at least 6 months before you go. There are also a lot of really good translation books out there.
As far as getting around the city, one day I decided to walk around by myself, Osaka is a big city like Tokyo, so I asked for directions to a store a few miles from my hotel. The girl at the desk gave me a map, all in Japanese of course (kanji), and she drew my course from the hotel to the store I needed to find. As I started out I thought that I was definately going to get lost, but as I walked I realized that as long as I stayed on the bigger streets I was OK. I also noticed that all I had to do to figure out which streets were which was to match the characters on the map to the street signs. Although I couldn't read the street names, it didn't matter because by matching the characters I new which street was which. Also a lot of the bigger street names are written in english letters, so that was helpful too.
Another thing about Osaka is that it is very flat, bicycle friendly and foot friendly. A lot of buses and underground subways, and I even heard about a bullet train that travels really fast! My point is that it is pretty easy to get around, especially the more you go out.
One thing I strongly suggest is familiarize yourself with Japanese food. I don't mean go out and eat at Japanese restaurants, I mean read books that can give you info. about traditional Japanese food, and what kinds of American foods are available. There are a lot of American fast food restaurants in Japan, and there are also a lot of American type convenience stores like am/pm. But I don't recommend living on that sort of food. There is a lot of really good authentic Japanese food, and some restaurants even will offer menues in English. If you're living there though I'm sure you're going to need to shop at grocery stores sometimes. Make sure to familiarize yourself with the food that they offer so that you can make the best of your experience there. I found a really good book, I think at Barnes and Noble or Borders, called "World Food Japan" by Lonely Planet. It even has really helpful phrases as well as a English-Japanese Glossary and a culinary dictionary in the back of the book. It's even a pretty small book, and only cost $13.99!
I'm sure you will still be nervous until you actually get there and realize it's not as scary as it seems. In regards to being isolated, it sounds like you will be able to meet other families that you will be able to communicate with. If the children will be in an english speaking school or program, then it sounds like you will have the opportunity to meet other families that you can relate to and may even be able to show you around. I think you will find that this will be an experience of a lifetime! I know I did! I hope this was helpful for you, good luck! The flight is long but well worth it!