Asif,
All unicode characters (including Japanese) can be encoded using UTF-8 (or UTF-16). Please use UTF-8 encoding, unless target system insists on using some other encoding.
If target system insists, they want file content to be encoded in Shift-JIS (please try Cp943, as it is supper set. Link).
Please use one of the 3 modules mentioned in Work with character encoding in SAP PI.