We have more than 300 hosts, and are adding new hosts almost daily. Hosts are spread all over Japan. Many speak English and welcome non Japanese speaking people. Hosts can be all kinds of places.  Many hosts accept WWOOFers all year round, provided you have a visa, and anyway, many are situated in warmer parts of Japan, so WWOOFing all year round is fine.
For current information about number and type of Hosts, see Host Preview