I don't know the language, but I know a bit about Japanese writing systems. There is no a general "Japanese" Unicode subset, but there are separate Unicode subsets for Hiragana
and their variants (supplements, extensions and half-width forms). Please see:
Using these articles (especially from http://Unicode.org
, Wikipedia for general information), recognize the Unicode subset in the form of code point ranges, put this data in your code and write then your method(s) bases on this data. It's quite simple.
I would suggest to wrote not a Boolean function (Japanese or not), but instead, a function returning some classifier showing what Japanese writing system a given character belongs to. Introduce an enumeration type to describe it.