The BeOS supports multibyte characters using the UTF-8 encoding format. Most web pages use the Shift-JIS format to encode Japanese characters.
For NetPositive to display Shift-JIS encoded pages, you must configure the mapping for converting between the two schemes. Instructions for doing so can be found in the BeOS User's Guide, Chapter 4 Using Internet Services, in the section Document Encoding.
You will also need to install a Unicode-enabled font, such as Bitstream's CyberBit, because none of the fonts installed with the BeOS include the extended character sets required to display kanji characters -- or other multi-byte glyphs, for that matter.
(Note: Be was unhappy with the quality of the hinting in most such fonts, including CyberBit, which causes characters at small sizes to look bad. We're still looking, and we hope to make available such a font in the future.)