Japanese language manuals have been posted to the Websydian WebClient documentation wiki. The translation was done by the Japanese WebClient distributor, Takaya Corporation.
If you are developing applications for languages other than English, you probably realize that this can be a challenge – particularly with eastern ideographic languages such as Japanese and Chinese. For a general overview of this, check out this System i Network article. For the technical details behind Unicode, check out this wikipedia article. Fortunately