Aioi Insurance in Tokyo offers both English assistance over the telephone as well as documentation and information in English.

I've used this company for 9 years. As well, a trusted Japanese friend said it's one of the better known insurance companies and the one which they use for their home.

Kamazawa-san
Aioi Insurance
03-3299-8640

Their rates seem reasonable to me.
Lea
http://home.catv-yokohama.ne.jp/zz/scooter/email.html