I am an Administrator on Japanese Wikipedia, and have initiated a discussion at Wikipedia:井戸端/subj/ビジュアルエディターに参照ツールとcitoidサービスを導入する提案 to enable Citoid on Japanese Wikipedia. The community is, in general, positive to enabling citoid, but there are a few concerns to be addressed.
When one inputs an ISBN or ISSN, citoid goes to WorldCat, an English site, to fetch the data. This does not give appropriate results for Japanese books. Taking ISBN 978-4-88367-243-1 as an example, the title of the book is 図書館の法令と政策, but WorldCat gives "Toshokan no horei to seisaku", an undesirable output. The suggestion is to use the NDL API, and if it is a non-Japanese book (dcterms:language field is not "jpn"), then fetch the data from WorldCat.
Some useful links as follows.
Specifications of NDL API: http://iss.ndl.go.jp/information/wp-content/uploads/2017/11/ndlsearch_api_all_20170329_en.pdf
Specifications of CiNii API: https://support.nii.ac.jp/en/cia/api/a_json
Zotero translator for CiNii: https://github.com/zotero/translators/blob/master/CiNii.js
Example of JSON api for cinii: https://twitter.com/jir_o/status/1067551874897764352