It seems Xapian is not prepared for Japanese, unfortunately.
https://public-inbox.org/meta/
20170702213657.GA5312@dcvr/
Core development list for the Ruby programming language
https://www.ruby-lang.org/en/community/mailing-lists/
+* https://public-inbox.org/ruby-dev/
+ ruby-dev@ruby-lang.org
+ Japanese development list for the Ruby programming language
+ https://www.ruby-lang.org/en/community/mailing-lists/
+
* https://public-inbox.org/sox-users/
sox-users@lists.sourceforge.net
Users' list for the SoX sound processing tool